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Document And Entity Information - shares	3 Months Ended
Document And Entity Information - shares	Mar. 31, 2020	May 07, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Entity Registrant Name	GOSSAMER BIO, INC.
Trading Symbol	GOSS
Entity Central Index Key	0001728117
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		66,358,439
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Security Exchange Name	NASDAQ
Entity File Number	001-38796
Entity Tax Identification Number	47-5461709
Entity Address, Address Line One	3013 Science Park Road
Entity Address, City or Town	San Diego
Entity Address, State or Province	CA
Entity Incorporation, State or Country Code	DE
Entity Address, Postal Zip Code	92121
City Area Code	858
Local Phone Number	684-1300
Document Quarterly Report	true
Document Transition Report	false

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 111,353	$ 135,089
Marketable securities	234,812	266,740
Prepaid expenses and other current assets	9,273	7,488
Total current assets	355,438	409,317
Property and equipment, net	5,669	5,425
Operating lease right-of-use assets	9,737	10,303
Other assets	634	1,559
Total assets	371,478	426,604
Current liabilities
Accounts payable	2,602	956
Accrued research and development expenses	16,797	19,258
Accrued expenses and other current liabilities	8,963	16,709
Total current liabilities	28,362	36,923
Long-term debt	28,548	28,459
Operating lease liabilities - long-term	8,105	8,737
Total liabilities	65,015	74,119
Commitments and contingencies
Stockholders' equity
Common stock, $0.0001 par value; 700,000,000 shares authorized as of March 31, 2020 and December 31, 2019; 66,338,201 shares issued and 62,094,312 shares outstanding as of March 31, 2020, and 66,284,003 shares issued and 61,635,477 shares outstanding as of December 31, 2019	7	7
Additional paid-in capital	695,205	686,390
Accumulated deficit	(388,244)	(334,170)
Accumulated other comprehensive income (loss)	(505)	258
Total stockholders' equity	306,463	352,485
Total liabilities and stockholders' equity	$ 371,478	$ 426,604

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	700,000,000	700,000,000
Common stock, shares, issued	66,338,201	66,284,003
Common stock, shares, outstanding	62,094,312	61,635,477

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating expenses:
Research and development	$ 41,414	$ 24,983
In process research and development	2,805	1,000
General and administrative	10,748	8,034
Total operating expenses	54,967	34,017
Loss from operations	(54,967)	(34,017)
Other income, net	893	1,406
Net loss	(54,074)	(32,611)
Other comprehensive income:
Foreign currency translation, net of tax	(87)
Other comprehensive income (loss)	(763)	140
Unrealized gain (loss) on marketable securities, net of tax	(676)	140
Comprehensive loss	$ (54,837)	$ (32,471)
Net loss per share, basic and diluted	$ (0.87)	$ (0.90)
Weighted average common shares outstanding, basic and diluted	61,890,323	36,317,230

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in-Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)	Series Seed Convertible Preferred Stock	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock
Beginning balance at Dec. 31, 2018	$ (120,069)	$ 2	$ 33,853	$ (153,863)	$ (61)
Temporary equity, beginning balance, shares at Dec. 31, 2018						20,000,000	45,714,286	71,506,513
Temporary equity, beginning balance at Dec. 31, 2018						$ 29,200	$ 79,615	$ 229,552
Beginning balance, shares at Dec. 31, 2018		8,051,418
Issuance of common stock in connection with a public offering,net of underwriting discounts,commissions,and offering costs	291,344	$ 2	291,342
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs, shares		19,837,500
Conversion of convertible preferred stock into common stock	338,367	$ 3	338,364			$ (29,200)	$ (79,615)	$ (229,552)
Convertible preferred stock, conversion of convertible preferred stock into common stock						(20,000,000)	(45,714,286)	(71,506,513)
Conversion of convertible preferred stock into common stock, shares		30,493,460
Vesting of restricted stock, shares		1,619,592
Stock-based compensation	3,089		3,089
Stock-based compensation, shares		27,500
Net loss	(32,611)			(32,611)
Other comprehensive income (loss)	140				140
Ending balance at Mar. 31, 2019	480,260	$ 7	666,648	(186,474)	79
Ending balance, shares at Mar. 31, 2019		60,029,470
Beginning balance at Dec. 31, 2019	352,485	$ 7	686,390	(334,170)	258
Beginning balance, shares at Dec. 31, 2019		61,635,477
Conversion of convertible preferred stock into common stock, shares		30,493,460
Vesting of restricted stock, shares		404,637
Exercise of stock options	$ 15		15
Exercise of stock options, shares	4,309	4,309
Stock-based compensation	$ 8,244		8,244	0
Issuance of common stock pursuant to Employee Stock Purchase Plan	556		556
Stock Issued During Period, Shares, Employee Stock Purchase Plans		49,889
Net loss	(54,074)			(54,074)
Other comprehensive income (loss)	(763)				(763)
Ending balance at Mar. 31, 2020	$ 306,463	$ 7	$ 695,205	$ (388,244)	$ (505)
Ending balance, shares at Mar. 31, 2020		62,094,312

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities
Net loss	$ (54,074)	$ (32,611)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	329	169
Stock-based compensation expense	8,244	3,089
In process research and development expenses	2,805	1,000
Amortization of operating lease right-of-use assets	566	565
Amortization of long-term debt discount and issuance costs	89
Amortization of premium on investments, net of accretion of discounts	(65)
Net realized gain on investments	(39)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,785)	(3,472)
Other assets	925	2,144
Operating lease liabilities	(566)	(504)
Accounts payable	1,609	2,132
Accrued expenses	(2,239)	(789)
Accrued research and development expenses	(2,461)	2,072
Accrued compensation and benefits	(5,633)	(1,569)
Net cash used in operating activities	(52,295)	(27,774)
Cash flows from investing activities
Research and development asset acquisitions, net of cash acquired	(2,805)	(1,000)
Purchase of marketable securities	(73,778)	(222,295)
Maturities of marketable securities	89,083	20,000
Sales of marketable securities	16,051	5,500
Purchase of property and equipment	(513)	(347)
Net cash provided by (used in) investing activities	28,038	(198,142)
Cash flows from financing activities
Proceeds from issuance of common stock		291,273
Proceeds from the exercise of stock options	15	71
Net cash provided by financing activities	571	291,344
Effect of exchange rate changes on cash and cash equivalents	(50)
Net increase (decrease) in cash and cash equivalents	(23,686)	65,428
Cash and cash equivalents, at the beginning of the period	135,089	105,419
Cash and cash equivalents, at the end of the period	111,353	170,847
Supplemental disclosure of cash flow information:
Cash paid for interest	618
Supplemental disclosure of noncash investing and financing activities:
Right-of-use assets obtained in exchange for lease liabilities		12,458
Change in unrealized gain (loss) on marketable securities, net of tax	(676)	140
Unpaid property and equipment	60	739
Conversion of convertible preferred stock to common stock		$ 338,367
Employee Stock Purchase Plan
Cash flows from financing activities
Proceeds from issuance of common stock	$ 556

Description of the Business

3 Months Ended

Mar. 31, 2020

Accounting Policies [Abstract]

Description of the Business

1. Description of the Business

Gossamer Bio, Inc. (including its subsidiaries, referred to as “we,” “us,” “our,”, or the “Company”) is a clinical-stage biopharmaceutical company focused on discovering, acquiring, developing and commercializing therapeutics in the disease areas of immunology, inflammation and oncology. The Company was incorporated in the state of Delaware on October 25, 2015 (originally as FSG Bio, Inc.) and is based in San Diego, California.

The condensed consolidated financial statements include the accounts of Gossamer Bio, Inc. and its wholly owned subsidiaries. All intercompany balances and transactions among the consolidated entity have been eliminated in consolidation.

Initial Public Offering in February 2019

On February 12, 2019, the Company completed its initial public offering (“IPO”) with the sale of 19,837,500 shares of common stock, including shares of common stock issued upon the exercise in full of the underwriters’ option to purchase additional shares, at a public offering price of $16.00 per share, resulting in net proceeds of $291.3 million, after deducting underwriting discounts, commissions, and offering expenses.

In addition, in connection with the completion of the IPO, all of the Company’s outstanding shares of convertible preferred stock were automatically converted into 30,493,460 shares of common stock.

Liquidity and Capital Resources

The Company has incurred significant operating losses since its inception. As of March 31, 2020, the Company had an accumulated deficit of $388.2 million. From the Company’s inception through March 31, 2020, the Company has funded its operations primarily through equity financings, including the Company’s IPO which closed on February 12, 2019. The Company raised $601.3 million from October 2017 through February 2019 through Series A and Series B Convertible Preferred Stock financings, a convertible note financing, and the IPO, after deducting underwriting discounts, commissions, and offering expenses. In addition, the Company received $12.8 million in cash in connection with the January 2018 acquisition of AA Biopharma Inc. On May 2, 2019 the Company, as guarantor, and its wholly-owned subsidiary GB001, Inc., as borrower, entered into a credit, guaranty and security agreement, as amended on September 18, 2019 (the “Credit Facility”), with MidCap Financial Trust (“MidCap”), as agent and lender, and the additional lenders party thereto from time to time (together with MidCap, the “Lenders”), pursuant to which the Lenders, including affiliates of MidCap and Silicon Valley Bank agreed to make term loans available to the Company for working capital and general business purposes, in a principal amount of up to $150.0 million in term loan commitments, including a $30.0 million term loan that was funded at the closing date. Under the Credit Facility, the Company has the ability to access the remaining $120.0 million in three additional tranches (of $40.0 million, $30.0 million and $50.0 million, respectively), subject to specified availability periods, the achievement of certain clinical development milestones, minimum cash requirements and other customary conditions. As of March 31, 2020, no other tranches under the Credit Facility have been drawn. See Note 5 for additional information regarding the Credit Facility.

The Company expects to continue to incur significant operating losses for the foreseeable future and may never become profitable. As a result, the Company will need to raise capital through equity offerings, debt financings and other capital sources, including potential collaborations, licenses and other similar arrangements. Management believes that it has sufficient working capital on hand to fund operations through at least the next twelve months from the date these condensed consolidated financial statements were available to be issued. There can be no assurance that the Company will be successful in acquiring additional funding, that the Company’s projections of its future working capital needs will prove accurate, or that any additional funding would be sufficient to continue operations in future years.

COVID-19

The COVID-19 outbreak has caused significant business disruption around the globe. The extent of the impact of COVID-19 on the Company’s operational and financial performance will depend on certain developments, including the duration and spread of the outbreak and the impact on the Company’s clinical trials, employees and vendors. At this point, the degree to which COVID-19 may impact the Company’s financial condition or results of operations is uncertain. A prolonged outbreak could have a material and adverse impact on financial results and business operations of the Company, including the timing and ability of Company to complete certain clinical trials and other efforts required to advance the development of its product candidates and raise additional capital. For example, due to the challenges of enrolling patients posed by the COVID-19 pandemic, the Company may experience delays in the commencement of and enrollment of patients in its planned Phase 2 clinical trial of GB002 in pulmonary arterial hypertension and planned Phase 2 clinical trial of GB004 in ulcerative colitis, as well as delays in reporting data results from its ongoing trials.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2020

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (“GAAP”) for interim financial information and with the instructions of the Securities and Exchange Commission (“SEC”) on Form 10-Q and Rule 10-01 of Regulation S-X. Accordingly, they do not include all of the information and disclosures required by GAAP for complete financial statements. In the opinion of management, the condensed consolidated financial statements include all adjustments necessary, which are of a normal and recurring nature, for the fair presentation of the Company’s financial position and of the results of operations and cash flows for the periods presented. These financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K filed with the SEC on March 24, 2020. The results of operations for the interim period shown in this report are not necessarily indicative of the results that may be expected for any other interim period or for the full year. The balance sheet at December 31, 2019, has been derived from the audited financial statements at that date.

Use of Estimates

The preparation of condensed consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of expenses during the reporting period. The most significant estimates in the Company’s condensed consolidated financial statements relate to accrued research and development expenses, the valuation of preferred and common stock, the valuation of stock options and the valuation allowance of deferred tax assets resulting from net operating losses. These estimates and assumptions are based on current facts, historical experience and various other factors believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities and the recording of expenses that are not readily apparent from other sources. Actual results could differ from those estimates.

Recently Adopted Accounting Pronouncements

In June 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2016-13, Financial Instruments-Credit Losses: Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”), which changes the impairment model for most financial assets and certain other instruments. For trade receivables and other instruments, entities will be required to use a new forward-looking expected loss model that generally will result in the earlier recognition of allowances for losses. For available-for-sale debt securities with unrealized losses, the losses will be recognized as allowances rather than as reductions in the amortized cost of the securities. This guidance is effective for annual reporting periods beginning after December 15, 2019, including interim periods within those years, with early adoption permitted only as of annual reporting periods beginning after December 15, 2018. The Company adopted ASU 2016-13 as of January 1, 2020. The adoption of this standard did not have a material impact on the Company’s condensed consolidated financial statements or related financial statement disclosures.

Net Loss Per Share

Basic net loss per share of common stock is computed by dividing net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net loss per share excludes the potential impact of the Company’s common stock options and unvested shares of restricted stock because their effect would be anti-dilutive due to the Company’s net loss. Since the Company had a net loss in each of the periods presented, basic and diluted net loss per common share are the same.

The table below provides potentially dilutive securities not included in the calculation of the diluted net loss per share because to do so would be anti-dilutive:

	As of March 31,
	2020		2019
Shares issuable upon exercise of stock options		10,192,073		7,469,973
Non-vested shares under restricted stock grants		5,395,550		5,862,440

Balance Sheet Accounts and Supplemental Disclosures

3 Months Ended

Mar. 31, 2020

Balance Sheets Accounts And Supplemental Disclosures [Abstract]

Balance Sheet Accounts and Supplemental Disclosures

3. Balance Sheet Accounts and Supplemental Disclosures

Property and Equipment

Property and equipment, net consisted of the following (in thousands):

	Estimated Useful Life (in years)	March 31, 2020		December 31, 2019
Office equipment	3-7	$	1,097	$	1,097
Computer equipment	5		131		124
Software	3		87		87
Lab equipment	2-5		3,614		3,054
Leasehold improvements	6-7		2,267		2,229
Construction in process	N/A		16		48
Total property and equipment			7,212		6,639
Less: accumulated depreciation			1,543		1,214
Property and equipment, net		$	5,669	$	5,425

Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):

	As of
	March 31, 2020		December 31, 2019
Accrued compensation	$	4,205	$	9,282
Operating lease liabilities, current		2,418		2,354
Accrued in process research and development		—		1,600
Accrued professional service fees		1,800		2,347
Accrued other		540		1,126
Total accrued expenses	$	8,963	$	16,709

Fair Value Measurements and Available for Sale Investments

3 Months Ended

Mar. 31, 2020

Fair Value Measurements And Available For Sale Investments [Abstract]

Fair Value Measurements and Available for Sale Investments

4. Fair Value Measurements and Available for Sale Investments

Fair Value Measurements

The accounting guidance defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the accounting guidance establishes a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value as follows:

Level 1: Observable inputs such as quoted prices in active markets;

Level 2: Inputs, other than the quoted prices in active markets, that are observable either directly or indirectly; and

Level 3: Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions.

The Company classifies its cash equivalents and available-for-sale investments within Level 1 or Level 2. The fair value of the Company’s investment grade corporate debt securities and commercial paper is determined using proprietary valuation models and analytical tools, which utilize market pricing or prices for similar instruments that are both objective and publicly available, such as matrix pricing or reported trades, benchmark yields, broker/dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, and offers.

Assets and Liabilities Measured at Fair Value on a Recurring Basis

The following table presents the hierarchy for assets measured at fair value on a recurring basis as of March 31, 2020 and December 31, 2019 (in thousands):

	Fair Value Measurements at End of Period Using:
				Quoted Market		Significant		Significant
				Prices for		Other Observable		Unobservable
		Total		Identical Assets		Inputs		Inputs
		Fair Value		(Level 1)		(Level 2)		(Level 3)
As of March 31, 2020
Money market funds		$	31,488	$	31,488	$	—	$	—
U.S. Treasury and agency securities			25,700		25,700		—		—
Commercial paper			71,961		—		71,961		—
Corporate debt securities			153,121		—		153,121		—
As of December 31, 2019
Money market funds		$	82,125	$	82,125	$	—	$	—
U.S. Treasury and agency securities			91,717		91,717		—		—
Commercial paper			37,411		—		37,411		—
Corporate debt securities			156,277		—		156,277		—

The Company did not reclassify any investments between levels in the fair value hierarchy during the periods presented.

Fair Value of Other Financial Instruments

As of March 31, 2020 and December 31, 2019, the carrying amounts of the Company’s financial instruments, which include cash, interest receivable, accounts payable and accrued expenses, approximate fair values because of their short maturities.

Interest receivable as of March 31, 2020 and December 31, 2019, was $1.3 million and $1.5 million, respectively, and is recorded as a component of prepaid expenses and other current assets on the condensed consolidated balance sheets.

The Company believes that its Credit Facility bears interest at a rate that approximates prevailing market rates for instruments with similar characteristics and, accordingly, the carrying value of the Credit Facility approximates fair value. The Company estimates the fair value of long-term debt utilizing an income approach. The Company uses a present value calculation to discount principal and interest payments and the final maturity payment on these liabilities using a discounted cash flow model based on observable inputs. The debt instrument is then discounted based on what the current market rates would be as of the reporting date. Based on the assumptions used to value these liabilities at fair value, the debt instrument is categorized as Level 2 in the fair value hierarchy.

Available for Sale Investments

The Company invests its excess cash in U.S. Treasury and agency securities and debt instruments of corporations and commercial obligations, which are classified as available-for-sale investments. These investments are carried at fair value and are included in the tables above. The Company evaluates securities with unrealized losses to determine whether such losses, if any, are due to credit-related factors. Realized gains and losses are calculated using the specific identification method and recorded as interest income or expense. The Company does not generally intend to sell the investments and it is not more likely than not that the Company will be required to sell the investments before recovery of their amortized cost bases, which may be at maturity.

The aggregate market value, cost basis, and gross unrealized gains and losses of available-for-sale investments by security type, classified in marketable securities and long-term investments as of March 31, 2020 are as follows (in thousands):

			Gross		Gross
	Amortized		Unrealized		Unrealized			Total
	Cost		Gains		Losses			Fair Value
Marketable securities
U.S. Treasury and agency securities	$	25,629	$	71	$	—		$	25,700
Commercial paper		55,991		—		—			55,991
Corporate debt securities		153,592		103		(574	)		153,121
Total marketable securities	$	235,212	$	174	$	(574	)	$	234,812

As of March 31, 2020, the Company classified $16.0 million of assets with original maturities of 90 days or less as cash equivalents. At each reporting date, the Company performs an evaluation of impairment to determine if any unrealized losses are due to credit-related factors. The Company records an allowance for credit losses when unrealized losses are due to credit-related factors. Factors considered when evaluating available-for-sale investments for impairment include the severity of the impairment, changes in underlying credit ratings, the financial condition of the issuer, the probability that the scheduled cash payments will continue to be made and the Company’s intent and ability to hold the investment until recovery of the amortized cost basis. The Company intends and has the ability to hold its investments in unrealized loss positions until their amortized cost basis has been recovered. As of March 31, 2020, there were no material declines in the market value of the Company’s available-for-sale investments due to credit-related factors.

Contractual maturities of available-for-sale debt securities, as of March 31, 2020, were as follows (in thousands):

	Estimated
	Fair Value
Due within one year	$	203,072
One to two years		31,740
Total	$	234,812

The Company has the ability, if necessary, to liquidate any of its cash equivalents and marketable securities to meet its liquidity needs in the next 12 months. Accordingly, those investments with contractual maturities greater than one year from the date of purchase are classified as current assets on the accompanying condensed consolidated balance sheets.

Long-Term Debt

3 Months Ended

Mar. 31, 2020

Debt Disclosure [Abstract]

Long-term Debt

5. Long-term Debt

On May 2, 2019, the Company entered into the Credit Facility described in Note 1, pursuant to which the Lenders agreed to make term loans available to the Company for working capital and general business purposes, in a principal amount of up to $150.0 million in term loan commitments, including a $30.0 million term loan that was funded at the closing date, with the ability to access the remaining $120.0 million in three additional tranches (of $40.0 million, $30.0 million and $50.0 million, respectively), subject to specified availability periods, the achievement of certain clinical development milestones, minimum cash requirements and other customary conditions. The second tranche is available no earlier than February 1, 2020 and no later than July 31, 2020. The third tranche is available no earlier than May 1, 2020 and no later than October 31, 2020. The fourth tranche is available no earlier than February 1, 2021 and no later than July 31, 2021. The Credit Facility is secured by substantially all of the Company’s and its domestic subsidiaries’ personal property, including intellectual property, and includes affirmative and negative covenants applicable to the Company.

Each term loan under the Credit Facility bears interest at an annual rate equal to the sum of (i) one-month LIBOR (customarily defined, with a change to prime rate if LIBOR funding becomes unlawful or impractical) plus (ii) 6.15%, subject to a LIBOR floor of 2.00%. The borrower is required to make interest-only payments on the term loan for all payment dates prior to June 1, 2021. The term loans under the Credit Facility will begin amortizing on June 1, 2021, with equal monthly payments of principal plus interest being made by the Company to the Lenders in consecutive monthly installments following such interest-only period for 36 months or, for any funding of the fourth tranche occurring after June 1, 2021, the number of months until the Credit Facility matures on May 1, 2024. Upon final repayment of the term loans, the borrower must pay an exit fee of 1.75% of the amount borrowed under the Credit Facility, less any partial exit fees previously paid. Upon partial prepayment of a portion of the term loans, the borrower must pay a partial exit fee of 1.75% of the principal being prepaid. At the borrower’s option, the borrower may prepay the outstanding principal balance of the term loan in whole or in part, subject to a prepayment fee of 3.0% of any amount prepaid if the prepayment occurs through and including the first anniversary of the closing date, 2.0% of the amount prepaid if the prepayment occurs after the first anniversary of the closing date through and including the second anniversary of the closing date, and 1.0% of any amount prepaid after the second anniversary of the closing date and prior to May 1, 2024.

The Credit Facility includes affirmative and negative covenants applicable to the Company and certain of its subsidiaries. The affirmative covenants include, among others, covenants requiring such entities to maintain their legal existence and governmental approvals, deliver certain financial reports, maintain insurance coverage, maintain property, pay taxes, satisfy certain requirements regarding accounts and comply with laws and regulations. The negative covenants include, among others, restrictions on such entities from transferring collateral, incurring additional indebtedness, engaging in mergers or acquisitions, paying dividends or making other distributions, making investments, creating liens, amending material agreements and organizational documents, selling assets and suffering a change in control, in each case subject to certain exceptions. The Company and certain of its subsidiaries are also subject to an ongoing minimum cash financial covenant in which they must maintain unrestricted cash in an amount not less than 25% of the outstanding principal amount of the term loans. As of March 31, 2020, the Company was in compliance with these covenants.

The Credit Facility also includes events of default, the occurrence and continuation of which could cause interest to be charged at the rate that is otherwise applicable plus 3.0% and would provide MidCap, as agent, with the right to exercise remedies against the Company and/or certain of its subsidiaries, and the collateral securing the Credit Facility, including foreclosure against the properties securing the credit facilities, including cash. These events of default include, among other things, failure to pay any amounts due under the Credit Facility, a breach of covenants under the Credit Facility, insolvency or the occurrence of insolvency events, the occurrence of a change in control, the occurrence of certain U.S. Food and Drug Administration (“FDA”) and regulatory events, failure to remain registered with the SEC and listed for trading on Nasdaq, the occurrence of a material adverse change, the occurrence of a default under a material agreement reasonably expected to result in a material adverse change, the occurrence of certain defaults under certain other indebtedness in an amount greater than $2,500,000 and the occurrence of certain defaults under subordinated indebtedness and convertible indebtedness.

Long-term debt as of March 31, 2020 consisted of the following (in thousands):

	March 31, 2020
Term loan	$	30,000
Debt discount and issuance costs		(1,452	)
Long-term debt	$	28,548

The scheduled future minimum principal payments are as follows (in thousands)

	March 31, 2020
2020 (remaining 9 months)		—
2021		5,833
2022		10,000
2023		10,000
2024		4,167
Total	$	30,000

Licenses, Asset Acquisitions and Contingent Consideration

3 Months Ended

Mar. 31, 2020

Business Combinations [Abstract]

Licenses, Asset Acquisitions and Contingent Consideration

6. Licenses, Asset Acquisitions and Contingent Consideration

The following purchased assets were accounted for as asset acquisitions as substantially all of the fair value of the assets acquired were concentrated in a group of similar assets and/or the acquired assets were not capable of producing outputs due to the lack of employees and early stage of development. Because the assets had not yet received regulatory approval, the fair value attributable to these assets was recorded as in process research and development (“IPR&D”) expenses in the Company’s condensed consolidated statement of operations for the three months ended March 31, 2020.

The Company accounts for contingent consideration payable upon achievement of certain regulatory, development or sales milestones in such asset acquisitions when the underlying contingency is met.

License from Pulmokine, Inc. (GB002)

On October 2, 2017, the Company, entered into a license agreement with Pulmokine, Inc. under which it was granted an exclusive worldwide license and sublicense to certain intellectual property rights owned or controlled by Pulmokine to develop and commercialize GB002 and certain backup compounds for the treatment, prevention and diagnosis of any and all disease or conditions. The Company also has the right to sublicense its rights under the license agreement, subject to certain conditions. The assets acquired are in the early stages of the FDA approval process, and the Company intends to further develop the assets acquired through potential FDA approval as evidenced by the milestone arrangement in the contract. The development activities cannot be performed without significant cost and effort by the Company. The agreement will remain in effect from the effective date, unless terminated earlier, until, on a licensed product-by-licensed product and country-by-country basis, the later of ten years from the date of first commercial sale or when there is no longer a valid patent claim covering such licensed product or specified regulatory exclusivity for the licensed product in such country. The Company is obligated to make future development and regulatory milestone payments of up to $63.0 million, commercial milestone payments of up to $45.0 million, and sales milestone payments of up to $190.0 million. The Company is also obligated to pay tiered royalties on sales for each licensed product, at percentages ranging from the mid-single digits to the high single-digits. The Company made an upfront payment of $5.5 million in October 2017. As of March 31, 2020, no milestones had been accrued as the underlying contingencies had not yet been met.

AA Biopharma Inc. Acquisition (GB001)

On January 4, 2018, the Company acquired AA Biopharma Inc. pursuant to a merger agreement, and with the acquisition acquired the rights to GB001 and certain backup compounds. In connection with the merger agreement, the Company issued an aggregate of 20,000,000 shares of Series Seed Convertible Preferred Stock and 1,101,278 shares of Common Stock to the AA Biopharma shareholders. The Company recorded IPR&D of $19.3 million in January 2018 in connection with the acquisition of AA Biopharma.

License from Aerpio Pharmaceuticals, Inc. (GB004)

On June 24, 2018, the Company entered into a license agreement with Aerpio Pharmaceuticals, Inc. (“Aerpio”) under which the Company was granted an exclusive worldwide license and sublicense to certain intellectual property rights owned or controlled by Aerpio to develop and commercialize GB004, and certain other related compounds for all applications. On May 11, 2020, the Company entered into an amendment to the license agreement with Aerpio pursuant to which the Company made an upfront payment of $15.0 million to Aerpio for a reduction in future milestone payments and royalties. Under the amended license agreement, the Company is obligated to make future approval milestone payments of up to $40.0 million and a sales milestone payment of $50.0 million. The Company also has the right to sublicense its rights under the license agreement, subject to certain conditions. The Company is also obligated to pay tiered royalties on sales for each licensed product, at percentages ranging from low- to mid-single digits, subject to certain customary reductions. Aerpio retains its twenty percent (20.0%) participation right on a disposition of GB004. The Company made an upfront payment of $20.0 million in June 2018, which represented the purchase consideration for an asset acquisition. As of March 31, 2020, no milestones had been accrued as the underlying contingencies had not yet been met.

Adhaere Pharmaceuticals, Inc. Acquisition (GB1275)

On September 21, 2018, the Company acquired Adhaere Pharmaceuticals, Inc. (“Adhaere”) pursuant to a merger agreement for an upfront payment of $7.5 million in cash, and with the acquisition acquired the rights to GB1275 and certain backup compounds. The Company is obligated to make regulatory, development and sales milestone payments of up to $62.0 million and pay tiered royalties on worldwide net sales, at percentages ranging from low to mid-single digits, subject to customary reductions. In September 2018, the Company recorded IPR&D of $7.5 million in connection with the acquisition of Adhaere. In May 2019, the Company made a milestone payment of $1.0 million in connection with the filing of the Investigational New Drug application for the GB1275 program. As of March 31, 2020, no other milestones had been accrued as the underlying contingencies had not yet been met.

The Company recorded the following IPR&D expense on the condensed consolidated statements of operations (in thousands):

	Three months ended March 31,
	2020		2019
GB001	$	—	$	—
GB004		—		—
GB1275		—		—
Other Programs		2,805		1,000
Total in process research and development	$	2,805	$	1,000

Stockholders' Equity

3 Months Ended

Mar. 31, 2020

Equity [Abstract]

Stockholders' Equity

7. Stockholders’ Equity

In connection with the Company’s IPO, the outstanding shares of the Company’s Series Seed, Series A, and Series B Convertible Preferred Stock automatically converted into 30,493,460 shares of common stock.

Common Stock

On December 3, 2015, the Company issued 9,160,888 shares of common stock as founder shares for services rendered to the Company, valued at $0.0001 par value per share, for a total of approximately $4,100. On January 4, 2018, incremental vesting conditions were placed on the previously issued founder shares. Fifty percent of the previously issued founder shares vested on January 4, 2018, and the remaining founder shares are subject to vesting restrictions over a period of five years.

Pursuant to the employment agreements with the Company’s founders executed January 4, 2018, the Company provided for certain potential additional issuances of common stock (the “anti-dilution shares”) to each of the founders to ensure the total number of shares of common stock held by them and their affiliates (inclusive of any shares subject to equity awards granted by the Company) would represent 15% of the Company’s fully-diluted capitalization until such time as the Company raised $300 million in equity capital, including the capital raised in the Series A financing.

In furtherance of this obligation, on May 21, 2018, the Company issued 251,547 shares of common stock to the founders for services rendered to the Company, valued at $2.61 per share with an additional 251,547 shares of restricted stock subject to the same vesting restrictions and vesting period as the founder shares. In addition, on September 6, 2018, the Company issued 1,795,023 shares of common stock to the founders for services rendered to the Company, valued at $9.63 per share, with an additional 1,795,023 shares of restricted stock subject to the same vesting restrictions and vesting period as the founder shares.

Each share of common stock is entitled to one vote. Common stock owners are entitled to dividends when funds are legally available and declared by the Board.

Shares of Common Stock Subject to Repurchase

In November 2017, in connection with the issuance of the Series A Convertible Preferred Stock, certain employees entered into stock restriction agreements, whereby 1,305,427 shares are subject to forfeiture by the Company upon the stockholder’s termination of employment or service to the Company. In January 2018, the Company’s founders entered into stock restriction agreements, whereby 4,580,444 of previously unrestricted shares of common stock were subject to service vesting conditions. These shares are also subject to forfeiture by the Company upon the stockholders’ termination of employment or service to the Company. Any shares subject to repurchase by the Company are not deemed, for accounting purposes, to be outstanding until those shares vest. As such, the Company recognizes the measurement date fair value of the restricted stock over the vesting period as compensation expense. As of March 31, 2020 and December 31, 2019, 4,243,889 and 4,648,526 shares of common stock were subject to repurchase by the Company, respectively. The unvested stock liability related to these awards is immaterial to all periods presented.

Equity Incentive Plans

3 Months Ended

Mar. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity Incentive Plans

8. Equity Incentive Plans

Approval of the 2019 Equity Incentive Plan

In January 2019, the Company’s board of directors and stockholders approved and adopted the 2019 Incentive Award Plan (the “2019 Plan”). The 2019 Plan became effective on February 6, 2019, the day prior to the effectiveness of the registration statement filed in connection with the IPO. Under the 2019 Plan, the Company may grant stock options, stock appreciation rights, restricted stock, restricted stock units, and other stock or cash-based awards to individuals who are then employees, officers, directors or consultants of the Company, and employees and consultants of the Company’s subsidiaries. A total of 5,750,000 shares of common stock were approved to be initially reserved for issuance under the 2019 Plan. The number of shares that remained available for issuance under the 2017 Plan (as defined below) as of the effective date of the 2019 Plan were, and shares subject to outstanding awards under the 2017 Plan as of the effective date of the 2019 Plan that are subsequently canceled, forfeited or repurchased by the Company will be added to the shares reserved under the 2019 Plan. In addition, the number of shares of common stock available for issuance under the 2019 Plan will be automatically increased on the first day of each calendar year during the ten-year term of the 2019 Plan, beginning with January 1, 2020 and ending with January 1, 2029, by an amount equal to 5% of the outstanding number of shares of the Company’s common stock on December 31 of the preceding calendar year or such lesser amount as determined by the Company’s board of directors. As of March 31, 2020, an aggregate of 2,232,426 shares of common stock were available for issuance under the 2019 Plan and 6,836,299 shares of common stock were subject to outstanding awards under the 2019 Plan.

Approval of the 2019 Employee Stock Purchase Plan

In January 2019, the Company’s board of directors and stockholders approved and adopted the 2019 Employee Stock Purchase Plan (the “ESPP”). The ESPP became effective as of February 6, 2019, the day prior to the effectiveness of the registration statement filed in connection with the IPO. The ESPP permits participants to purchase common stock through payroll deductions of up to 20% of their eligible compensation. A total of 700,000 shares of common stock were approved to be initially reserved for issuance under the ESPP. In addition, the number of shares of common stock available for issuance under the ESPP will be automatically increased on the first day of each calendar year during the first ten-years of the term of the ESPP, beginning with January 1, 2020 and ending with January 1, 2029, by an amount equal to 1% of the outstanding number of shares of the Company’s common stock on December 31 of the preceding calendar year or such lesser amount as determined by the Company’s board of directors. During the three months ended March 31, 2020, 49,889 shares were issued pursuant to the ESPP. As of March 31, 2020, an aggregate of 1,312,951 shares of common stock were available for issuance under the ESPP.

2017 Equity Incentive Plan

The Company’s 2017 Equity Incentive Plan (the “2017 Plan”) permitted the granting of incentive stock options, non-statutory stock options, restricted stock, restricted stock units and other stock-based awards. Subsequent to the adoption of the 2019 Plan, no additional equity awards can be made under the 2017 Plan. As of March 31, 2020, 4,507,435 shares of common stock were subject to outstanding options under the 2017 Plan, and 488,580 shares of restricted stock awards granted under the 2017 plan were unvested.

Stock Options

The fair value of each employee and non-employee stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. The Company, prior to the closing of its IPO on February 12, 2019, was a private company and lacked company-specific historical and implied volatility information. Therefore, it estimated its expected volatility based on the historical volatility of a publicly traded set of peer companies. Due to the lack of historical exercise history, the expected term of the Company’s stock options for employees has been determined utilizing the “simplified” method for awards. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is zero based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

The following table summarizes stock option activity during the three months ended March 31, 2020:

	Shares Subject to Options Outstanding					Weighted- Average
				Weighted-		Remaining
				Average Exercise		Contractual Life		Aggregate
	Shares			Price		(Years)		Intrinsic Value
								(in thousands)
Outstanding as of December 31, 2019		8,538,060		$	13.67		9.0	$	35,385
Options granted		1,725,222		$	14.55
Option exercised		(4,309	)	$	3.38
Options forfeited/cancelled		(66,900	)	$	15.50
Outstanding as of March 31, 2020		10,192,073		$	13.81		9.0	$	12,052
Options vested and exercisable as of March 31, 2020		2,146,119		$	11.38		8.6	$	4,843

The aggregate intrinsic value in the above table is calculated as the difference between fair value of the Company’s common stock price on March 31, 2020 and the exercise price of the stock options.

The weighted-average grant date fair value per share for the stock option grants during the three months ended March 31, 2020 was $10.42.

The aggregate fair value of stock options that vested during the three months ended March 31, 2020 was $10.3 million.

Restricted Stock

The summary of the Company’s restricted stock activity is as follows:

	Number of			Weighted-
	Restricted			Average
	Stock Units			Grant Date
	Outstanding			Fair Value
Nonvested at December 31, 2019		4,648,526		$	3.98
Granted		1,151,661		$	11.98
Vested		(404,637	)	$	3.71
Forfeited		—			—
Nonvested at March 31, 2020		5,395,550		$	5.71

At March 31, 2020, the total unrecognized compensation related to unvested restricted stock awards granted was $25.8 million, which the Company expects to recognize over a weighted-average period of approximately 2.7 years.

Stock-Based Compensation Expense

Stock-based compensation expense has been reported in the Company’s condensed consolidated statements of operations as follows (in thousands):

	Three months ended
	March 31,
	2020		2019
Research and development	$	4,586	$	1,293
General and administrative		3,658		1,796
Total stock-based compensation	$	8,244	$	3,089

At March 31, 2020, the total unrecognized compensation related to unvested stock option awards granted was $72.0 million, which the Company expects to recognize over a weighted-average period of approximately 2.9 years.

As of March 31, 2020, total unrecognized compensation expense related to the ESPP was $2.0 million, which the Company expects to recognize over a weighted-average period of approximately 1.2 years.

Commitments and Contingencies

3 Months Ended

Mar. 31, 2020

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

9. Commitments and Contingencies

Leases

The Company subleases certain office and laboratory space under a non-cancelable operating lease expiring in January 2025 for the initial leased space and December 2022 for expansion space leased pursuant to an amendment to the lease agreement entered into in August 2018. The sublease agreement included options to extend for the entire premises through October 2028. The options to extend must be exercised prior to the termination of the original lease agreement. The period covered by the options was not included in the non-cancellable lease term as it not was not determined to be reasonably certain to be executed. The lease agreement also includes a one-time termination option for the expansion space only whereby the Company can terminate the lease with advance written notice. The termination option was not determined to be reasonably certain to be executed. The lease is subject to charges for common area maintenance and other costs, and base rent is subject to an annual 3% increase each subsequent year. Costs determined to be variable and not based on an index or rate were not included in the measurement of the operating lease liabilities.

In November 2019, the Company entered into an additional non-cancelable lease agreement for certain office and laboratory space (the “permanent space”) in San Diego, California, commencing on May 1, 2020 and expiring on December 31, 2021. The lease agreement includes a lease for temporary space commencing on January 1, 2020 and expiring on the commencement date of the lease of the permanent space. The monthly base rent for the permanent and temporary space is $63,425 and $28,745, respectively. The lease agreement included an option to extend the term of the permanent space for twelve months. The option to extend must be exercised nine months prior to the termination of the original lease agreement. The period covered by the option was not included in the non-cancellable lease term as it not was not determined to be reasonably certain to be executed. The lease is subject to charges for common area maintenance and other costs, and base rent is subject to an annual 3% increase each subsequent year.

Monthly rent expense is recognized on a straight-line basis over the term of the leases. The operating leases are included in the balance sheet at the present value of the lease payments at a weighted-average discount rate of 7% using the rate of interest that the Company would have to pay to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment as the leases do not provide an implicit rate. The weighted average remaining lease term was 4.0 years.

Lease costs were comprised of the following (in thousands):

	Three months ended March 31, 2020
Operating lease cost	$	753
Short-term lease cost		22
Total lease cost	$	775

Cash paid for amounts included in the measurement of operating lease liabilities for the three months ended March 31, 2020 was $0.8 million.

Gross future minimum annual rental commitments as of March 31, 2020, were as follows (in thousands):

	Undiscounted Rent Payments
Year ending December 31,
2020 (remaining 9 months)		2,285
2021		3,127
2022		3,220
2023		1,694
2024		1,745
Total undiscounted rent payments	$	12,071

Present value discount		(1,548	)
Present value	$	10,523
Current portion of operating lease liability (included as a component of accrued expenses)	$	2,418
Noncurrent operating lease liabilities		8,105
Total operating lease liability	$	10,523

For the three months ended March 31, 2020 the Company recorded approximately $0.8 million, in rent expense.

Litigation

Kuhne vs. Gossamer Bio, Inc., et. al.

On April 3, 2020, Scott Kuhne, individually and on behalf of all others similarly situated, filed a putative class action lawsuit against the Company, certain of its executive officers and directors, and the underwriters of its IPO in the United States District Court for the Southern District of California (Case No. 3:20-cv-00649-DMS-MDD). The complaint was filed on behalf of all persons who purchased or otherwise acquired the Company’s securities between February 8, 2019 and December 13, 2019. The complaint alleges that the Company, certain of its executive officers and directors, and the underwriters of its IPO made false and/or misleading statements and failed to disclose material adverse facts about its business, operations and prospects in violation of Sections 11, 12(a)(2) and 15 of the Securities Act of 1933, as amended, and Sections 10(b) (and Rule 10b-5 promulgated thereunder) and 20(a) of the Securities Exchange Act of 1934, as amended. The plaintiff seeks damages, interest, costs, attorneys’ fees, and other unspecified equitable relief. The Company intends to vigorously defend this matter. Given the uncertainty of litigation, the preliminary stage of the case, and the legal standards that must be met for, among other things, class certification and success on the merits, the Company cannot estimate the reasonably possible loss or range of loss that may result from this action.

Subsequent Events

3 Months Ended

Mar. 31, 2020

Subsequent Events [Abstract]

Subsequent Events

10. Subsequent Events

On May 11, 2020, the Company entered into an amendment to the license agreement with Aerpio pursuant to which the Company made an upfront payment of $15.0 million to Aerpio for a reduction in future milestone payments and royalties. Under the amended license agreement, the Company is obligated to make future approval milestone payments of up to $40.0 million and a sales milestone payment of $50.0 million. The royalties on worldwide net sales now range from a low- to mid-single digit percentage of net sales. The Company continues to be responsible for the remaining development, regulatory, and commercialization expenses for GB004, and Aerpio’s 20.0% participation right on a disposition of GB004 remains.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2020

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Recently Adopted Accounting Pronouncements

Net Loss Per Share

The table below provides potentially dilutive securities not included in the calculation of the diluted net loss per share because to do so would be anti-dilutive:

	As of March 31,
	2020		2019
Shares issuable upon exercise of stock options		10,192,073		7,469,973
Non-vested shares under restricted stock grants		5,395,550		5,862,440

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2020

Accounting Policies [Abstract]

Schedule of Potentially Dilutive Securities not Included in Calculation of Diluted Net Loss Per Share

The table below provides potentially dilutive securities not included in the calculation of the diluted net loss per share because to do so would be anti-dilutive:

	As of March 31,
	2020		2019
Shares issuable upon exercise of stock options		10,192,073		7,469,973
Non-vested shares under restricted stock grants		5,395,550		5,862,440

Balance Sheet Accounts and Supplemental Disclosures (Tables)

3 Months Ended

Mar. 31, 2020

Balance Sheets Accounts And Supplemental Disclosures [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):

	Estimated Useful Life (in years)	March 31, 2020		December 31, 2019
Office equipment	3-7	$	1,097	$	1,097
Computer equipment	5		131		124
Software	3		87		87
Lab equipment	2-5		3,614		3,054
Leasehold improvements	6-7		2,267		2,229
Construction in process	N/A		16		48
Total property and equipment			7,212		6,639
Less: accumulated depreciation			1,543		1,214
Property and equipment, net		$	5,669	$	5,425

Schedule of Accrued Expenses

Accrued expenses and other current liabilities consisted of the following (in thousands):

	As of
	March 31, 2020		December 31, 2019
Accrued compensation	$	4,205	$	9,282
Operating lease liabilities, current		2,418		2,354
Accrued in process research and development		—		1,600
Accrued professional service fees		1,800		2,347
Accrued other		540		1,126
Total accrued expenses	$	8,963	$	16,709

Fair Value Measurements and Available for Sale Investments (Tables)

3 Months Ended

Mar. 31, 2020

Fair Value Measurements And Available For Sale Investments [Abstract]

Assets Measured at Fair Value on Recurring Basis

The following table presents the hierarchy for assets measured at fair value on a recurring basis as of March 31, 2020 and December 31, 2019 (in thousands):

	Fair Value Measurements at End of Period Using:
				Quoted Market		Significant		Significant
				Prices for		Other Observable		Unobservable
		Total		Identical Assets		Inputs		Inputs
		Fair Value		(Level 1)		(Level 2)		(Level 3)
As of March 31, 2020
Money market funds		$	31,488	$	31,488	$	—	$	—
U.S. Treasury and agency securities			25,700		25,700		—		—
Commercial paper			71,961		—		71,961		—
Corporate debt securities			153,121		—		153,121		—
As of December 31, 2019
Money market funds		$	82,125	$	82,125	$	—	$	—
U.S. Treasury and agency securities			91,717		91,717		—		—
Commercial paper			37,411		—		37,411		—
Corporate debt securities			156,277		—		156,277		—

Schedule of Available for Sale Investments by Security Type

			Gross		Gross
	Amortized		Unrealized		Unrealized			Total
	Cost		Gains		Losses			Fair Value
Marketable securities
U.S. Treasury and agency securities	$	25,629	$	71	$	—		$	25,700
Commercial paper		55,991		—		—			55,991
Corporate debt securities		153,592		103		(574	)		153,121
Total marketable securities	$	235,212	$	174	$	(574	)	$	234,812

Schedule of Contractual Maturities of Available-for-sale Debt Securities

Contractual maturities of available-for-sale debt securities, as of March 31, 2020, were as follows (in thousands):

	Estimated
	Fair Value
Due within one year	$	203,072
One to two years		31,740
Total	$	234,812

Long-Term Debt (Tables)

3 Months Ended

Mar. 31, 2020

Debt Disclosure [Abstract]

Schedule of Long-Term Debt

Long-term debt as of March 31, 2020 consisted of the following (in thousands):

	March 31, 2020
Term loan	$	30,000
Debt discount and issuance costs		(1,452	)
Long-term debt	$	28,548

Schedule of Future Minimum Principal Payments

The scheduled future minimum principal payments are as follows (in thousands)

	March 31, 2020
2020 (remaining 9 months)		—
2021		5,833
2022		10,000
2023		10,000
2024		4,167
Total	$	30,000

Licenses, Asset Acquisitions and Contingent Consideration (Tables)

3 Months Ended

Mar. 31, 2020

Business Combinations [Abstract]

Schedule of IPR&D Expense

The Company recorded the following IPR&D expense on the condensed consolidated statements of operations (in thousands):

	Three months ended March 31,
	2020		2019
GB001	$	—	$	—
GB004		—		—
GB1275		—		—
Other Programs		2,805		1,000
Total in process research and development	$	2,805	$	1,000

Equity Incentive Plans (Tables)

3 Months Ended

Mar. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

The following table summarizes stock option activity during the three months ended March 31, 2020:

	Shares Subject to Options Outstanding					Weighted- Average
				Weighted-		Remaining
				Average Exercise		Contractual Life		Aggregate
	Shares			Price		(Years)		Intrinsic Value
								(in thousands)
Outstanding as of December 31, 2019		8,538,060		$	13.67		9.0	$	35,385
Options granted		1,725,222		$	14.55
Option exercised		(4,309	)	$	3.38
Options forfeited/cancelled		(66,900	)	$	15.50
Outstanding as of March 31, 2020		10,192,073		$	13.81		9.0	$	12,052
Options vested and exercisable as of March 31, 2020		2,146,119		$	11.38		8.6	$	4,843

Summary of Restricted Stock Activity

The summary of the Company’s restricted stock activity is as follows:

	Number of			Weighted-
	Restricted			Average
	Stock Units			Grant Date
	Outstanding			Fair Value
Nonvested at December 31, 2019		4,648,526		$	3.98
Granted		1,151,661		$	11.98
Vested		(404,637	)	$	3.71
Forfeited		—			—
Nonvested at March 31, 2020		5,395,550		$	5.71

Stock-Based Compensation Expense Reported in Condensed Consolidated Statements of Operations

Stock-based compensation expense has been reported in the Company’s condensed consolidated statements of operations as follows (in thousands):

	Three months ended
	March 31,
	2020		2019
Research and development	$	4,586	$	1,293
General and administrative		3,658		1,796
Total stock-based compensation	$	8,244	$	3,089

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2020

Commitments And Contingencies Disclosure [Abstract]

Schedule of Lease Costs

Lease costs were comprised of the following (in thousands):

	Three months ended March 31, 2020
Operating lease cost	$	753
Short-term lease cost		22
Total lease cost	$	775

Schedule of Gross Future Minimum Annual Rental Commitments

Gross future minimum annual rental commitments as of March 31, 2020, were as follows (in thousands):

	Undiscounted Rent Payments
Year ending December 31,
2020 (remaining 9 months)		2,285
2021		3,127
2022		3,220
2023		1,694
2024		1,745
Total undiscounted rent payments	$	12,071

Present value discount		(1,548	)
Present value	$	10,523
Current portion of operating lease liability (included as a component of accrued expenses)	$	2,418
Noncurrent operating lease liabilities		8,105
Total operating lease liability	$	10,523

Description of the Business - Additional Information (Detail)				17 Months Ended
	May 02, 2019 USD ($) Tranche	Feb. 12, 2019 USD ($) $ / shares shares	Jan. 04, 2018 USD ($)	Feb. 28, 2019 USD ($)	Mar. 31, 2020 USD ($) Tranche	Dec. 31, 2019 USD ($)
Description Of Business [Line Items]
Accumulated deficit					$ (388,244,000)	$ (334,170,000)
Funds raised through Series A and Series B Convertible Preferred Stock, convertible note financings and completed IPO				$ 601,300,000
Term Loan
Description Of Business [Line Items]
Maximum borrowing capacity	$ 150,000,000
Amount funded	$ 30,000,000
Number of additional tranches executed \| Tranche	3				0
Remaining borrowing capacity	$ 120,000,000
Term Loan \| Tranche One
Description Of Business [Line Items]
Remaining borrowing capacity	40,000,000
Term Loan \| Tranche Two
Description Of Business [Line Items]
Remaining borrowing capacity	30,000,000
Term Loan \| Tranche Three
Description Of Business [Line Items]
Remaining borrowing capacity	$ 50,000,000
AA Biopharma Inc.
Description Of Business [Line Items]
Cash received in connection with acquisition			$ 12,800,000
Initial Public Offering ("IPO")
Description Of Business [Line Items]
Sale of shares of common stock \| shares		19,837,500
Offering price, per share \| $ / shares		$ 16.00
Net proceeds, after deducting underwriting discounts, commissions, and offering expenses		$ 291,300,000
Initial Public Offering ("IPO") \| Conversion of Convertible Preferred Stock into Common Stock
Description Of Business [Line Items]
Common stock issued in conversion of convertible preferred stock \| shares		30,493,460

Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities not Included in Calculation of Diluted Net Loss Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Shares Issuable upon Exercise of Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities not included from calculation of diluted net loss per share	10,192,073	7,469,973
Non-vested Shares under Restricted Stock Grants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities not included from calculation of diluted net loss per share	5,395,550	5,862,440

Balance Sheet Accounts and Supplemental Disclosures - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Total property and equipment	$ 7,212	$ 6,639
Less: accumulated depreciation	1,543	1,214
Property and equipment, net	5,669	5,425
Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 1,097	1,097
Office Equipment \| Minimum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	3 years
Office Equipment \| Maximum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	7 years
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 131	124
Property and equipment, estimated useful life	5 years
Software
Property Plant And Equipment [Line Items]
Total property and equipment	$ 87	87
Property and equipment, estimated useful life	3 years
Lab Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 3,614	3,054
Lab Equipment \| Minimum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	2 years
Lab Equipment \| Maximum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	5 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	$ 2,267	2,229
Leasehold Improvements \| Minimum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	6 years
Leasehold Improvements \| Maximum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	7 years
Construction in Process
Property Plant And Equipment [Line Items]
Total property and equipment	$ 16	$ 48

Balance Sheet Accounts and Supplemental Disclosures - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Accrued compensation	$ 4,205	$ 9,282
Operating lease liabilities, current	2,418	2,354
Accrued in process research and development		1,600
Accrued professional service fees	1,800	2,347
Accrued other	540	1,126
Total accrued expenses	$ 8,963	$ 16,709

Fair Value Measurements and Available for Sale Investments - Assets Measured at Fair Value on Recurring Basis (Details) - Recurring Basis - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 31,488	$ 82,125
U S Treasury and Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	25,700	91,717
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	71,961	37,411
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	153,121	156,277
Quoted Market Prices for Identical Assets (Level 1) \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	31,488	82,125
Quoted Market Prices for Identical Assets (Level 1) \| U S Treasury and Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	25,700	91,717
Significant Other Observable Inputs (Level 2) \| Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	71,961	37,411
Significant Other Observable Inputs (Level 2) \| Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 153,121	$ 156,277

Fair Value Measurements and Available for Sale Investments - Additional Information (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value hierarchy level 1 to level 2	$ 0	$ 0
Fair value hierarchy level 2 to level 1	0	0
Cash Equivalents, at Carrying Value	16,000,000
Prepaid Expenses and Other Current Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Interest and securities receivable	$ 1,300,000	$ 1,500,000

Fair Value Measurements and Available for Sale Investments - Schedule of Available for sale Investments by Security Type (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Investments, Amortized Cost	$ 235,212
Available-for-sale Investments, Gross Unrealized Gains	174
Available-for-sale Investments, Gross Unrealized Losses	(574)
Available-for-sale Investments, Total Fair Value	234,812
U S Treasury and Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Investments, Amortized Cost	25,629
Available-for-sale Investments, Gross Unrealized Gains	71
Available-for-sale Investments, Total Fair Value	25,700
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Investments, Amortized Cost	55,991
Available-for-sale Investments, Total Fair Value	55,991
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Investments, Amortized Cost	153,592
Available-for-sale Investments, Gross Unrealized Gains	103
Available-for-sale Investments, Gross Unrealized Losses	(574)
Available-for-sale Investments, Total Fair Value	$ 153,121

Fair Value Measurements and Available for Sale Investments - Schedule of Contractual Maturities of Available-for-sale Debt Securities (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Fair Value Measurements And Available For Sale Investments [Abstract]
Due within one year	$ 203,072
One to two years	31,740
Total	$ 234,812

Long-Term Debt - Additional Information (Details) - Term Loan		3 Months Ended
	May 02, 2019 USD ($) Tranche	Mar. 31, 2020 USD ($)
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 150,000,000
Amount funded	$ 30,000,000
Number of additional tranches \| Tranche	3
Remaining borrowing capacity	$ 120,000,000
Interest only payment period	36 months
Maturity date	May 01, 2024
Percentage of exit fee on amount borrowed on final repayment	1.75%
Percentage of exit fee on amount borrowed on partial prepayment	1.75%
Debt covenant minimum unrestricted cash percentage	25.00%
Minimum indebtedness amount		$ 2,500,000
Prepayment Occurs through First Anniversary of Closing Date
Debt Instrument [Line Items]
Percentage of prepayment fee	3.00%
First Anniversary of Closing Date through Second Anniversary of Closing Date
Debt Instrument [Line Items]
Percentage of prepayment fee	2.00%
Second Anniversary of Closing Date and Prior to May 1, 2024
Debt Instrument [Line Items]
Percentage of prepayment fee	1.00%
LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	6.15%
Interest rate	2.00%
Debt instrument, description of variable rate basis	Each term loan under the Credit Facility bears interest at an annual rate equal to the sum of (i) one-month LIBOR (customarily defined, with a change to prime rate if LIBOR funding becomes unlawful or impractical) plus (ii) 6.15%, subject to a LIBOR floor of 2.00%.
MidCap Financial Trust
Debt Instrument [Line Items]
Basis spread on variable rate		3.00%
Tranche One
Debt Instrument [Line Items]
Remaining borrowing capacity	$ 40,000,000
Tranche Two
Debt Instrument [Line Items]
Remaining borrowing capacity	30,000,000
Tranche Three
Debt Instrument [Line Items]
Remaining borrowing capacity	$ 50,000,000

Long-Term Debt - Schedule of Long-term Debt (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Debt Disclosure [Abstract]
Term loan	$ 30,000
Debt discount and issuance costs	(1,452)
Long-term debt	$ 28,548

Long-Term Debt - Schedule of Future Minimum Principal Payments (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Debt Disclosure [Abstract]
2021	$ 5,833
2022	10,000
2023	10,000
2024	4,167
Total	$ 30,000

Licenses, Asset Acquisitions and Contingent Consideration - Additional Information (Details) - USD ($)										1 Months Ended	3 Months Ended
	May 11, 2020	Sep. 30, 2018	Sep. 21, 2018	Jun. 30, 2018	Jun. 24, 2018	Jan. 31, 2018	Jan. 04, 2018	Oct. 31, 2017	Oct. 02, 2017	May 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Business Acquisition [Line Items]
In process research and development											$ 2,805,000	$ 1,000,000
Pulmokine, Inc. \| License Agreement \| GB002
Business Acquisition [Line Items]
Product license term									10 years
Upfront payment								$ 5,500,000
Milestones accrued											0
Pulmokine, Inc. \| License Agreement \| GB002 \| Maximum
Business Acquisition [Line Items]
Development and regulatory milestone payments, payable									$ 63,000,000
Commercial milestone payments, payable									45,000,000
Sales milestone payments, payable									$ 190,000,000
AA Biopharma Inc. \| GB001
Business Acquisition [Line Items]
In process research and development						$ 19,300,000
AA Biopharma Inc. \| GB001 \| Common Stock
Business Acquisition [Line Items]
Business acquisition, aggregate number of shares issued							1,101,278
AA Biopharma Inc. \| GB001 \| Series Seed Convertible Preferred Stock
Business Acquisition [Line Items]
Business acquisition, aggregate number of shares issued							20,000,000
Aerpio Pharmaceuticals, Inc. \| License Agreement \| GB004
Business Acquisition [Line Items]
Milestones accrued											0
Upfront payment, purchase consideration paid				$ 20,000,000
Participation rights					20.00%
Aerpio Pharmaceuticals, Inc. \| License Agreement \| GB004 \| Subsequent Event
Business Acquisition [Line Items]
Upfront payment	$ 15,000,000
Aerpio Pharmaceuticals, Inc. \| License Agreement \| GB004 \| Maximum \| Subsequent Event
Business Acquisition [Line Items]
Development and regulatory milestone payments, payable	40,000,000
Sales milestone payments, payable	$ 50,000,000
Adhaere Pharmaceuticals, Inc. \| GB1275
Business Acquisition [Line Items]
Milestones accrued											$ 0
In process research and development		$ 7,500,000
Upfront payment, purchase consideration paid			$ 7,500,000
Milestones payment										$ 1,000,000
Adhaere Pharmaceuticals, Inc. \| GB1275 \| Maximum
Business Acquisition [Line Items]
Regulatory, development and sales milestone payments payable			$ 62,000,000

Licenses, Asset Acquisitions and Contingent Consideration - Schedule of IPR&D Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Acquired Finite Lived Intangible Assets [Line Items]
Total in process research and development	$ 2,805	$ 1,000
Other Programs
Acquired Finite Lived Intangible Assets [Line Items]
Total in process research and development	$ 2,805	$ 1,000

Stockholders' Equity - Additional Information (Details) - USD ($)					1 Months Ended		3 Months Ended		12 Months Ended
	Sep. 06, 2018	May 21, 2018	Jan. 04, 2018	Dec. 03, 2015	Jan. 31, 2018	Nov. 30, 2017	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Stockholders Equity [Line Items]
Common stock, par value							$ 0.0001		$ 0.0001
Issuance of common stock in connection with a public offering net of underwriting discounts, commissions and offering costs								$ 291,344,000
Common stock, voting right							one
Shares of common stock, repurchase							4,243,889		4,648,526
Series A Convertible Preferred Stock \| Stock Restriction Agreements
Stockholders Equity [Line Items]
Shares subject to forfeiture						1,305,427
Founder
Stockholders Equity [Line Items]
Common stock issued shares	1,795,023	251,547
Percentage of fully diluted share capital			15.00%
Increased in equity capital amount			$ 300,000,000
Common stock issued price per share	$ 9.63	$ 2.61
Additional shares of restricted stock subject to vesting restrictions	1,795,023	251,547
Common Stock
Stockholders Equity [Line Items]
Conversion of convertible preferred stock into common stock, shares							30,493,460	30,493,460
Common stock issued shares								19,837,500
Issuance of common stock in connection with a public offering net of underwriting discounts, commissions and offering costs								$ 2,000
Common Stock \| Stock Restriction Agreements
Stockholders Equity [Line Items]
Unrestricted shares of common stock were subject to service vesting conditions					4,580,444
Founder Shares
Stockholders Equity [Line Items]
Common stock issued shares				9,160,888
Common stock, par value				$ 0.0001
Issuance of common stock in connection with a public offering net of underwriting discounts, commissions and offering costs				$ 4,100
Common Stock vesting percentage			50.00%
Common stock vesting period			5 years

Equity Incentive Plans - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Feb. 06, 2019	Mar. 31, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares outstanding awarded		10,192,073	8,538,060
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield		0.00%
Weighted-average grant date fair value per share		$ 10.42
Aggregate fair value of stock options vested during period		$ 10.3
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock awards granted		1,151,661
Unrecognized compensation costs		$ 25.8
Weighted-average period of cost expects to recognize		2 years 8 months 12 days
Stock-Based Compensation Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average period of cost expects to recognize		2 years 10 months 24 days
Unrecognized compensation expense related to the ESPP.		$ 72.0
2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of share reserved for future issuance	5,750,000
Term of awards	10 years
Percentage of amount increase in outstanding shares	5.00%
Common stock available for issuance		2,232,426
Shares outstanding awarded		6,836,299
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of share reserved for future issuance	700,000
Term of awards	10 years
Percentage of amount increase in outstanding shares	1.00%
Number of share issued under ESPP		49,889
Number of share reserved for future issuance		1,312,951
2019 Employee Stock Purchase Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase of common stock through payroll deductions, percentage	20.00%
2017 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares outstanding awarded		4,507,435
2017 Equity Incentive Plan \| Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock awards granted		488,580
Twenty Thousand And Twenty Stock Purchase Plan \| Stock-Based Compensation Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average period of cost expects to recognize		1 year 2 months 12 days
Unrecognized compensation expense related to the ESPP.		$ 2.0

Equity Incentive Plans - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Subject to Options Outstanding, Outstanding as of December 31, 2019	8,538,060
Shares Subject to Options Outstanding, Options granted	1,725,222
Shares Subject to Options Outstanding, Option exercised	(4,309)
Shares Subject to Options Outstanding, Options forfeited/cancelled	(66,900)
Shares Subject to Options Outstanding, Outstanding as of March 31,2020	10,192,073	8,538,060
Shares Subject to Options Outstanding, Options vested and exercisable as of March 31, 2020	2,146,119
Weighted-Average Exercise Price, Outstanding as of December 31, 2019	$ 13.67
Weighted-Average Exercise Price, Options granted	14.55
Weighted-Average Exercise Price, Option exercised	3.38
Weighted-Average Exercise Price, Options forfeited/cancelled	15.50
Weighted-Average Exercise Price, Outstanding as of March 31, 2020	13.81	$ 13.67
Weighted-Average Exercise Price, Options vested and exercisable as of March 31,2020	$ 11.38
Weighted-Average Remaining Contractual Life (Years), Outstanding as of December 31, 2019	9 years	9 years
Weighted-Average Remaining Contractual Life (Years), Options vested and exercisable as of March 31, 2020	8 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding as of 31 Dec, 2019	$ 12,052	$ 35,385
Aggregate Intrinsic Value, Options vested and exercisable	$ 4,843

Equity Incentive Plans - Summary of Restricted Stock Activity (Details) - Restricted Stock	3 Months Ended
	Mar. 31, 2020 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Restricted Stock Units Outstanding, Nonvested \| shares	4,648,526
Number of Restricted Stock Units Outstanding, Granted \| shares	1,151,661
Number of Restricted Stock Units Outstanding, Vested \| shares	(404,637)
Number of Restricted Stock Units Outstanding, Forfeited \| shares	0
Number of Restricted Stock Units Outstanding, Nonvested \| shares	5,395,550
Weighted-Average Grant Date Fair Value, Nonvested \| $ / shares	$ 3.98
Weighted-Average Grant Date Fair Value, Granted \| $ / shares	11.98
Weighted-Average Grant Date Fair Value, Vested \| $ / shares	3.71
Weighted-Average Grant Date Fair Value, Forfeited \| $ / shares	0
Weighted-Average Grant Date Fair Value, Nonvested \| $ / shares	$ 5.71

Equity Incentive Plans - Stock-Based Compensation Expense Reported in Condensed Consolidated Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 8,244	$ 3,089
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	4,586	1,293
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 3,658	$ 1,796

Commitments and Contingencies - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended
	Nov. 30, 2019	Mar. 31, 2020
Non-cancelable operating sublease, expiration date for initial leased space		2025-01
Non-cancelable operating sublease, expiration date for expansion space leased		2022-12
Lessee, Operating Sublease, Existence of Option to Extend [true false]		true
Lessee, operating sublease, option to extend		options to extend for the entire premises through October 2028
Operating sublease, annual increase percentage for base rent		3.00%
Monthly base rent for permanent space	$ 63,425
Monthly base rent for temporary space	$ 28,745
Option to extend lease term of permanent space	12 months
Annual increase percentage for lease charges	3.00%
Lessee, operating lease, discount rate		7.00%
Weighted average remining lease term		4 years
Cash paid for operating lease liabilities		$ 800,000
Rent expense		$ 800,000
San Diego, California
Lease commencement date for permanent space	May 01, 2020
Lease expiration date for permanent space	Dec. 31, 2021
Lease commencement date for temporary space	Jan. 01, 2020

Commitments and Contingencies - Schedule of Lease Costs (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Leases [Abstract]
Operating lease cost	$ 753
Short-term lease cost	22
Total lease cost	$ 775

Commitments and Contingencies - Schedule of Gross Future Minimum Annual Rental Commitments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Leases [Abstract]
2020 (remaining 9 months)	$ 2,285
2021	3,127
2022	3,220
2023	1,694
2024	1,745
Total undiscounted rent payments	12,071
Present value discount	(1,548)
Present value	10,523
Current portion of operating lease liability (included as a component of accrued expenses)	2,418	$ 2,354
Noncurrent operating lease liabilities	8,105	$ 8,737
Total operating lease liability	$ 10,523

Subsequent Events - Additional Information (Details) - Aerpio Pharmaceuticals, Inc. - License Agreement - GB004 - Subsequent Event	May 11, 2020 USD ($)
Business Acquisition [Line Items]
Upfront payment	$ 15,000,000
Participation right on disposition	20.00%
Maximum
Business Acquisition [Line Items]
Development and regulatory milestone payments, payable	$ 40,000,000
Sales milestone payments, payable	$ 50,000,000