Document And Entity Information - shares	3 Months Ended
Document And Entity Information - shares	Mar. 31, 2019	May 08, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
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 Common Stock, Shares Outstanding		65,893,276

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 170,847	$ 105,219
Marketable securities	310,374	123,439
Restricted cash		200
Prepaid expenses and other current assets	16,567	3,095
Total current assets	497,788	231,953
Property and equipment, net	4,110	3,193
Operating lease right-of-use assets	11,922
Other assets	2,129	4,273
Total assets	515,949	239,419
Current liabilities
Accounts payable	4,548	2,182
Accrued research and development expenses	12,725	10,653
Accrued expenses	7,879	7,568
Total current liabilities	25,152	20,403
Operating lease liabilities	10,537
Accrued expenses - long-term		718
Total liabilities	35,689	21,121
Commitments and contingencies - Note 10
Stockholders' equity (deficit)
Common stock, $0.0001 par value; 700,000,000 shares authorized as of March 31, 2019 and 49,160,177 shares authorized as of December 31, 2018; 65,891,910 shares issued and 60,029,470 shares outstanding as of March 31, 2019, and 15,533,450 shares issued and 8,051,418 shares outstanding as of December 31, 2018	7	2
Additional paid-in capital	666,648	33,853
Accumulated deficit	(186,474)	(153,863)
Accumulated other comprehensive income (loss)	79	(61)
Total stockholders' equity (deficit)	480,260	(120,069)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 515,949	239,419
Series Seed Convertible Preferred Stock
Current liabilities
Convertible preferred stock, shares issued, outstanding, and liquidation preference		29,200
Series A Convertible Preferred Stock
Current liabilities
Convertible preferred stock, shares issued, outstanding, and liquidation preference		79,615
Series B Convertible Preferred Stock
Current liabilities
Convertible preferred stock, shares issued, outstanding, and liquidation preference		$ 229,552

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	700,000,000	49,160,177
Common stock, shares, issued	65,891,910	15,533,450
Common stock, shares, outstanding	60,029,470	8,051,418
Series Seed Convertible Preferred Stock
Temporary equity, par value	$ 0.0001	$ 0.0001
Temporary equity, shares issued	0	20,000,000
Temporary equity, shares outstanding	0	20,000,000
Temporary equity, liquidation preference	$ 0	$ 20,000
Series A Convertible Preferred Stock
Temporary equity, par value	$ 0.0001	$ 0.0001
Temporary equity, shares issued	0	45,714,286
Temporary equity, shares outstanding	0	45,714,286
Temporary equity, liquidation preference	$ 0	$ 80,000
Series B Convertible Preferred Stock
Temporary equity, par value	$ 0.0001	$ 0.0001
Temporary equity, shares issued	0	71,506,513
Temporary equity, shares outstanding	0	71,506,513
Temporary equity, liquidation preference	$ 0	$ 230,000

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses:
Research and development	$ 24,983	$ 2,624
In process research and development	1,000	20,898
General and administrative	8,034	2,604
Total operating expenses	34,017	26,126
Loss from operations	(34,017)	(26,126)
Other income (expenses)
Interest income	1,049	89
Interest expense	(19)
Other income	376
Total other income, net	1,406	89
Net loss	(32,611)	(26,037)
Other comprehensive income:
Unrealized gain on marketable securities, net of tax	140
Other comprehensive loss	140
Comprehensive loss	$ (32,471)	$ (26,037)
Net loss per share, basic and diluted	$ (0.90)	$ (4.49)
Weighted average common shares outstanding, basic and diluted	36,317,230	5,797,693

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (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, 2017	$ (6,862)		$ 32	$ (6,894)
Temporary equity, beginning balance, shares at Dec. 31, 2017						0		0
Temporary equity, beginning balance at Dec. 31, 2017						$ 0		$ 0
Beginning balance, shares at Dec. 31, 2017		9,160,888
Issuance of Series A preferred stock for cash, net of $0.4 million in offering costs							$ 71,944
Issuance of Series A preferred stock for cash, net of $0.4 million in offering costs, shares							41,328,286
Temporary equity, issuance of stock for acquisition						$ 29,200
Temporary equity, issuance of stock for acquisition, shares						20,000,000
Issuance of stock for acquisition	2,874		2,874
Issuance of stock for acquisition. shares		1,101,278
Issuance of Series A preferred stock to convert debt and accrued interest							$ 6,124
Issuance of Series A preferred stock to convert debt and accrued interest, shares							3,499,209
Stock-based compensation	605		605
Incremental vesting conditions place on previously issued common shares		(4,580,444)
Net loss	(26,037)			(26,037)
Ending balance at Mar. 31, 2018	(29,420)		3,511	(32,931)
Temporary equity, ending balance, shares at Mar. 31, 2018						20,000,000	44,827,495
Temporary equity, ending balance at Mar. 31, 2018						$ 29,200	$ 78,068
Ending balance, shares at Mar. 31, 2018		5,681,722
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
Convertible preferred stock, conversion of convertible preferred stock into common stock, shares						(20,000,000)	(45,714,286)	(71,506,513)
Convertible preferred stock, conversion of convertible preferred stock into common stock						$ (29,200)	$ (79,615)	$ (229,552)
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	140				140
Ending balance at Mar. 31, 2019	$ 480,260	$ 7	$ 666,648	$ (186,474)	$ 79
Temporary equity, ending balance, shares at Mar. 31, 2019						0	0	0
Ending balance, shares at Mar. 31, 2019		60,029,470

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities
Net loss	$ (32,611)	$ (26,037)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	169	6
Stock-based compensation expense	3,089	605
In process research and development expenses	1,000	20,898
Changes in operating assets and liabilities:
Operating lease right of use assets and liabilities, net	61
Prepaid expenses and other current assets	(3,472)	(94)
Other Assets	2,144	(425)
Accounts payable	2,132	546
Accrued expenses	(789)	441
Accrued research and development expenses	2,072	(126)
Accrued compensation and benefits	(1,569)	593
Accrued interest expense		(115)
Net cash used in operating activities	(27,774)	(3,708)
Cash flows from investing activities
Research and development asset acquisitions, net of cash acquired	(1,000)	11,176
Purchase of investments	(222,295)
Sales and maturities of investments	25,500
Purchase of property and equipment	(347)	(511)
Net cash (used in) provided by investing activities	(198,142)	10,665
Cash flows from financing activities
Proceeds from issuance of common stock in a public offering, net	291,273
Proceeds from the exercise of stock options	71
Proceeds from issuance of Series A convertible preferred stock, net		71,944
Repayment of notes payable to related parties		(40)
Net cash provided by financing activities	291,344	71,904
Net increase in cash, cash equivalents and restricted cash	65,428	78,861
Cash, cash equivalents and restricted cash, at the beginning of the period	105,419	315
Cash, cash equivalents and restricted cash, at the end of the period	170,847	79,176
Supplemental disclosure of noncash investing and financing activities:
Acquisition of in-process research and development through issuance of stock		19,284
Issuance of Series A convertible preferred stock to convert debt and accrued interest		6,124
Recognition of operating lease right of use asset	12,458
Recognition of operating lease liabilities	13,182
Conversion of convertible preferred stock to common stock	338,367
Change in unrealized gain on marketable securities, net of tax	140
Unpaid property and equipment	$ 739	$ 43

Description of the Business

3 Months Ended

Mar. 31, 2019

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.

Stock Split

In January 2019, the board of directors of the Company approved a reverse stock split of the Company’s common stock at a ratio of one for every 4.5 shares previously held. The reverse stock split became effective on January 23, 2019. All share and per share data included in these condensed consolidated financial statements reflect the stock split.

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, 2019, the Company had an accumulated deficit of $186.5 million. From the Company’s inception through March 31, 2019, 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 March 2019 through Series A and Series B Convertible Preferred Stock, convertible note financings, and the completed 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. 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 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.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2019

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, 2018 included in the Company’s Annual Report on Form 10-K filed with the SEC on March 22, 2019. 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, 2018, 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

The Company adopted Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) (“ASC 842”), as of January 1, 2019, using the optional transition method. The optional transition method provides a method for recording existing leases at adoption and a cumulative catch up adjustment on January 1, 2019 for any differences between ASC 842 and the legacy guidance provided in Accounting Standards Codification 840, Leases that would have impacted our income statement. No retrospective restatements are required under the optional transition method. In addition, the Company elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed us to carry forward the historical lease classification. The Company also applied the short-term lease recognition exemption for leases with terms at inception not greater than 12 months.

Adoption of the new standard resulted in the recording of additional operating lease right-of-use assets and operating lease liabilities of approximately $12.5 million and $13.2 million, respectively, as of January 1, 2019. The difference between the operating lease right-of-use assets and lease liabilities are due to accrued deferred rent and unamortized lease incentives.

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 Series Seed Convertible Preferred Stock, Series A Convertible Preferred Stock, and Series B Convertible Preferred Stock, 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,
	2019		2018
Shares issuable upon conversion of Series Seed Convertible Preferred Stock		—		4,444,444
Shares issuable upon conversion of Series A Convertible Preferred Stock		—		9,961,663
Shares issuable upon exercise of stock options		7,469,973		—
Non-vested shares under restricted stock grants		5,862,440		5,885,865

Balance Sheet Accounts and Supplemental Disclosures

3 Months Ended

Mar. 31, 2019

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, 2019		December 31, 2018
Office equipment	3-7	$	664	$	918
Computer equipment	5		33		15
Software	3		50		50
Lab equipment	2-5		1,870		1,070
Leasehold improvements	6-7		1,314		1,243
Construction in process	N/A		645		194
Total property and equipment			4,576		3,490
Less: accumulated depreciation			466		297
Property and equipment, net		$	4,110	$	3,193

Accrued Expenses

Accrued expenses consisted of the following (in thousands):

	As of
	March 31, 2019		December 31, 2018
Accrued compensation	$	2,533	$	4,102
Operating lease liabilities		2,170		—
Accrued professional service fees		1,318		2,697
Accrued other		1,858		769
Total accrued expenses	$	7,879	$	7,568

Fair Value Measurements and Available for Sale Investments

3 Months Ended

Mar. 31, 2019

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.

We classify our cash equivalents and available-for-sale investments within Level 1 or Level 2. The fair value of our 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, 2019 and December 31, 2018 (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, 2019
Money market funds		$	40,965	$	40,965	$	—	$	—
U.S. Treasury securities			199,874		199,874		—		—
Commercial paper			26,687		—		26,687		—
Corporate debt securities			83,813		—		83,813		—
As of December 31, 2018
Money market funds		$	17,295	$	17,295	$	—	$	—
U.S. Treasury securities			123,439		123,439		—		—

The Company did not reclassify and investments between levels in the fair value hierarchy during the first quarter of 2019 or 2018.

Fair Value of Other Financial Instruments

As of March 31, 2019 and December 31, 2018, the carrying amounts of the Company’s financial instruments, which include cash, interest and securities receivable, accounts payable and accrued expenses, approximate fair values because of their short maturities.

Interest and securities receivable as of March 31, 2019 and December 31, 2018, was $11.2 million and $0.6 million, respectively, and is recorded as a component of prepaid expenses and other current assets on the condensed consolidated balance sheets. Securities receivable reflect the timing differences of maturities or settlements of investments and the ultimate reinvestment of such amounts.

Available for Sale Investments

We invest our excess cash in U.S. Treasury securities and debt instruments of corporations and commercial obligations, which we classify 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 other than temporary. Realized gains and losses are calculated using the specific identification method and recorded as interest income or expense. We do not generally intend to sell the investments and it is not more likely than not that we 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 for the three months ended March 31, 2019 are as follows (in thousands):

			Gross		Gross
	Amortized		Unrealized		Unrealized			Total
	Cost		Gains		Losses			Fair Value
Marketable securities
U.S. Treasury securities	$	199,811	$	69	$	(6	)	$	199,874
Commercial paper		26,694		—		(7	)		26,687
Corporate debt securities		83,738		84		(9	)		83,813
Total marketable securities	$	310,243	$	153	$	(22	)	$	310,374

None of the investments have been in a gross unrealized loss for a period greater than 12 months. At each reporting date, we perform an evaluation of impairment to determine if any unrealized losses are other-than-temporary. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, and our intent and ability to hold the investment until recovery of the amortized cost basis. We intend and have the ability to hold our investments in unrealized loss positions until their amortized cost basis has been recovered. Further, based on our evaluation, we determined that unrealized losses were not other-than-temporary at March 31, 2019 and December 31, 2018.

Contractual maturities of available-for-sale debt securities, as of March 31, 2019, were as follows (in thousands):

	Estimated
	Fair Value
Due within one year	$	261,450
One to two years		48,924
Total	$	310,374

We have the ability, if necessary, to liquidate any of our cash equivalents and short-term investments to meet our 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.

Convertible Note Financing

3 Months Ended

Mar. 31, 2019

Debt Disclosure [Abstract]

Convertible Note Financing

5. Convertible Note Financing

On October 2, 2017, the Company issued a convertible promissory note (the “Note”) in an amount of $6.0 million to an investor. The Note accrued interest at 8% per year and had a maturity date of October 2, 2018. The Note was subject to an automatic conversion upon a qualified equity financing defined as a raise of $40.0 million, excluding the conversion of the Note and other indebtedness. The conversion was equal to the outstanding principal amount of the Note plus all accrued and previously unpaid interest thereon, divided by the lowest price per share paid by investor for qualified equity financing. On January 4, 2018, the Note converted into 3,499,209 shares of Series A Convertible Preferred Stock.

Asset Acquisitions and Contingent Consideration

3 Months Ended

Mar. 31, 2019

Business Combinations [Abstract]

Asset Acquisitions and Contingent Consideration

6. 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, 2019.

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.

Acquisition of 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 U.S. Food and Drug Administration (“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, which was recorded as IPR&D. As of March 31, 2019, 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.

Acquisition of 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. The Company also has the right to sublicense its rights under the license agreement, subject to certain conditions. The Company is obligated to make future development and regulatory milestone payments of up to $55.0 million, commercial milestone payments of up to $85.0 million and sales milestone payments of up to $260.0 million. The Company is also obligated to pay tiered royalties on sales for each licensed product, at percentages ranging from a high single-digit to mid-teens, subject to certain customary reductions. 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, 2019, 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 future regulatory, development and sales milestones 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. As of March 31, 2019, no 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,
	2019		2018
GB001	$	—	$	19,148
Other Programs		1,000		1,750
Total in process research and development	$	1,000	$	20,898

Stockholders' Equity (Deficit)

3 Months Ended

Mar. 31, 2019

Equity [Abstract]

Stockholders’ Equity (Deficit)

7. Stockholders’ Equity (Deficit)

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, 2019 and December 31, 2018 shares of common stock subject to repurchase by the Company was 5,862,450 shares and 7,482,032 shares, respectively. The unvested stock liability related to these awards is immaterial to all periods presented.

Equity Incentive Plans

3 Months Ended

Mar. 31, 2019

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 of the effective date of the 2019 Plan 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 31st of the preceding calendar year or such lesser amount as determined by the Company’s board of directors. As of March 31, 2019, an aggregate of 3,359,856 shares of common stock were available for issuance under the 2019 Plan and 2,390,144 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 31st of the preceding calendar year or such lesser amount as determined by the Company’s board of directors. As of March 31, 2019, an aggregate of 700,000 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.

At March 31, 2019, 6,285,478 shares of common stock were subject to outstanding awards under the 2017 Plan.

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 IPO on February 12, 2019, was a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates 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, 2019:

	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, 2018		5,107,329		$	7.51		9.7	$	16,343
Options granted		2,390,144		$	21.74
Option exercised		(27,500	)	$	2.61
Options forfeited/cancelled		—
Outstanding as of March 31, 2019		7,469,973		$	12.08		9.6	$	72,708
Options vested and exercisable as of March 31, 2019		160,682		$	3.20		8.9	$	2,968

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, 2019 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, 2019 was $21.74. At March 31, 2019, the total unrecognized compensation related to unvested stock option awards granted was $55.7 million, which the Company expects to recognize over a weighted-average period of approximately 3.5 years.

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, 2018		7,482,032		$	4.01
Granted		—			—
Vested		(1,619,592	)	$	4.31
Forfeited		—			—
Nonvested at March 31, 2019		5,862,440		$	3.92

At March 31, 2019, the total unrecognized compensation related to unvested restricted stock awards granted was $16.9 million, which the Company expects to recognize over a weighted-average period of approximately 3.8 years.

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,
	2019		2018
Research and development	$	1,293	$	4
General and administrative		1,796		601
Total stock-based compensation	$	3,089	$	605

For the three months ended March 31, 2019 and 2018, $3.1 million and $0.6 million, respectively, of the stock-based compensation expense related to the issuance of the anti-dilution shares.

Commitments and Contingencies

3 Months Ended

Mar. 31, 2019

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 December 2024 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.

Monthly rent expense is recognized on a straight-line basis over the term of the lease. The operating lease is included in the balance sheet at the present value of the lease payments at a 7% discount rate 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 5.0 years.

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

	Three months ended March 31, 2019

Operating lease cost	$	753
Variable lease cost		395
Short-term lease cost		14
Total lease cost	$	1,162

Cash paid for amounts included in the measurement of operating lease liabilities for the three months ended March 31, 2019 was $1.2 million.

Gross future minimum annual rental commitments as of March 31, 2019, were as follows (in thousands):

	Undiscounted Rent Payments
Year ending December 31,
2019 (remaining 9 months)	$	2,216
2020		3,035
2021		3,123
2022		3,216
2023		1,690
2024		1,741
Total undiscounted rent payments	$	15,021

Present value discount		(2,314	)
Present value	$	12,707

Current portion of operating lease liability (included as a component of Accrued expenses)	$	2,170
Noncurrent operating lease liabilities		10,537
Total operating lease liability	$	12,707

Future minimum lease payments under non-cancelable operating leases at December 31, 2018 were as follows (in thousands):

Years ending December 31,
2019	$	2,944
2020		3,035
2021		3,123
2022		3,216
2023		1,690
Thereafter		1,741
	$	15,749

For the three months ended March 31, 2018 the Company recorded approximately $0.5 million in rent expense.

Litigation

The Company is not a party to any material legal proceedings and is not aware of any pending or threatened claims. From time to time, the Company may be subject to various legal proceedings and claims that arise in the ordinary course of its business activities.

Subsequent Events

3 Months Ended

Mar. 31, 2019

Subsequent Events [Abstract]

Subsequent Events

10. Subsequent Events

On May 2, 2019, the Company and its wholly-owned subsidiary GB001, Inc., as borrower, entered into a credit, guaranty and security agreement (the “Credit Facility”) agented by MidCap Financial Trust (“MidCap”) 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 which 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 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 Company 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 Company 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 Company must pay a partial exit fee of 1.75% of the principal being prepaid. At the Company’s option, the Company 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.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2019

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,
	2019		2018
Shares issuable upon conversion of Series Seed Convertible Preferred Stock		—		4,444,444
Shares issuable upon conversion of Series A Convertible Preferred Stock		—		9,961,663
Shares issuable upon exercise of stock options		7,469,973		—
Non-vested shares under restricted stock grants		5,862,440		5,885,865

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2019

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,
	2019		2018
Shares issuable upon conversion of Series Seed Convertible Preferred Stock		—		4,444,444
Shares issuable upon conversion of Series A Convertible Preferred Stock		—		9,961,663
Shares issuable upon exercise of stock options		7,469,973		—
Non-vested shares under restricted stock grants		5,862,440		5,885,865

Balance Sheet Accounts and Supplemental Disclosures (Tables)

3 Months Ended

Mar. 31, 2019

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, 2019		December 31, 2018
Office equipment	3-7	$	664	$	918
Computer equipment	5		33		15
Software	3		50		50
Lab equipment	2-5		1,870		1,070
Leasehold improvements	6-7		1,314		1,243
Construction in process	N/A		645		194
Total property and equipment			4,576		3,490
Less: accumulated depreciation			466		297
Property and equipment, net		$	4,110	$	3,193

Schedule of Accrued Expenses

Accrued expenses consisted of the following (in thousands):

	As of
	March 31, 2019		December 31, 2018
Accrued compensation	$	2,533	$	4,102
Operating lease liabilities		2,170		—
Accrued professional service fees		1,318		2,697
Accrued other		1,858		769
Total accrued expenses	$	7,879	$	7,568

Fair Value Measurements and Available for Sale Investments (Tables)

3 Months Ended

Mar. 31, 2019

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, 2019 and December 31, 2018 (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, 2019
Money market funds		$	40,965	$	40,965	$	—	$	—
U.S. Treasury securities			199,874		199,874		—		—
Commercial paper			26,687		—		26,687		—
Corporate debt securities			83,813		—		83,813		—
As of December 31, 2018
Money market funds		$	17,295	$	17,295	$	—	$	—
U.S. Treasury securities			123,439		123,439		—		—

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 securities	$	199,811	$	69	$	(6	)	$	199,874
Commercial paper		26,694		—		(7	)		26,687
Corporate debt securities		83,738		84		(9	)		83,813
Total marketable securities	$	310,243	$	153	$	(22	)	$	310,374

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

Contractual maturities of available-for-sale debt securities, as of March 31, 2019, were as follows (in thousands):

	Estimated
	Fair Value
Due within one year	$	261,450
One to two years		48,924
Total	$	310,374

Asset Acquisitions and Contingent Consideration (Tables)

3 Months Ended

Mar. 31, 2019

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,
	2019		2018
GB001	$	—	$	19,148
Other Programs		1,000		1,750
Total in process research and development	$	1,000	$	20,898

Equity Incentive Plans (Tables)

3 Months Ended

Mar. 31, 2019

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, 2019:

	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, 2018		5,107,329		$	7.51		9.7	$	16,343
Options granted		2,390,144		$	21.74
Option exercised		(27,500	)	$	2.61
Options forfeited/cancelled		—
Outstanding as of March 31, 2019		7,469,973		$	12.08		9.6	$	72,708
Options vested and exercisable as of March 31, 2019		160,682		$	3.20		8.9	$	2,968

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, 2018		7,482,032		$	4.01
Granted		—			—
Vested		(1,619,592	)	$	4.31
Forfeited		—			—
Nonvested at March 31, 2019		5,862,440		$	3.92

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,
	2019		2018
Research and development	$	1,293	$	4
General and administrative		1,796		601
Total stock-based compensation	$	3,089	$	605

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Schedule of Lease Costs

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

	Three months ended March 31, 2019

Operating lease cost	$	753
Variable lease cost		395
Short-term lease cost		14
Total lease cost	$	1,162

Schedule of Gross Future Minimum Annual Rental Commitments

Gross future minimum annual rental commitments as of March 31, 2019, were as follows (in thousands):

	Undiscounted Rent Payments
Year ending December 31,
2019 (remaining 9 months)	$	2,216
2020		3,035
2021		3,123
2022		3,216
2023		1,690
2024		1,741
Total undiscounted rent payments	$	15,021

Present value discount		(2,314	)
Present value	$	12,707

Current portion of operating lease liability (included as a component of Accrued expenses)	$	2,170
Noncurrent operating lease liabilities		10,537
Total operating lease liability	$	12,707

Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases

Future minimum lease payments under non-cancelable operating leases at December 31, 2018 were as follows (in thousands):

Years ending December 31,
2019	$	2,944
2020		3,035
2021		3,123
2022		3,216
2023		1,690
Thereafter		1,741
	$	15,749

Description of the Business - Additional Information (Detail) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended	18 Months Ended
	Feb. 12, 2019 USD ($) $ / shares shares	Jan. 04, 2018 USD ($)	Jan. 31, 2019	Mar. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Description Of Business [Line Items]
Reverse stock split				one for every 4.5 shares
Reverse stock spilt conversion ratio for common stock			0.0022
Accumulated deficit				$ (186,474)	$ (186,474)	$ (153,863)
Funds raised through Series A and Series B Convertible Preferred Stock, convertible note financings and completed IPO					$ 601,300
AA Biopharma Inc.
Description Of Business [Line Items]
Cash received in connection with acquisition		$ 12,800
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
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 - Additional Information (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating lease right-of-use assets	$ 11,922
Operating lease liabilities	$ 12,707
Accounting Standards Update 2016-02
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating lease right-of-use assets		$ 12,500
Operating lease liabilities		$ 13,200

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	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Shares Issuable upon Conversion of Series Seed Convertible
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities not included from calculation of diluted net loss per share		4,444,444
Shares Issuable upon Conversion of Series A Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities not included from calculation of diluted net loss per share		9,961,663
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	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	5,862,440	5,885,865

Balance Sheet Accounts and Supplemental Disclosures - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 4,576	$ 3,490
Less: accumulated depreciation	466	297
Property and equipment, net	4,110	3,193
Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 664	918
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	$ 33	15
Property and equipment, estimated useful life	5 years
Software
Property Plant And Equipment [Line Items]
Total property and equipment	$ 50	50
Property and equipment, estimated useful life	3 years
Lab Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 1,870	1,070
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	$ 1,314	1,243
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	$ 645	$ 194

Balance Sheet Accounts and Supplemental Disclosures - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued compensation	$ 2,533	$ 4,102
Operating lease liabilities	2,170
Accrued professional service fees	1,318	2,697
Accrued other	1,858	769
Total accrued expenses	$ 7,879	$ 7,568

Fair Value Measurements and Available for Sale Investments - Assets Measured at Fair Value on Recurring Basis (Details) - Recurring Basis - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 40,965	$ 17,295
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	199,874	123,439
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	26,687
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	83,813
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	40,965	17,295
Quoted Market Prices for Identical Assets (Level 1) \| U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	199,874	$ 123,439
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	26,687
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	$ 83,813

Fair Value Measurements and Available for Sale Investments - Additional Information (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
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
Prepaid Expenses and Other Current Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Interest and securities receivable	$ 11,200,000	$ 600,000

Fair Value Measurements and Available for Sale Investments - Schedule of Available for sale Investments by Security Type (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Investments, Amortized Cost	$ 310,243
Available-for-sale Investments, Gross Unrealized Gains	153
Available-for-sale Investments, Gross Unrealized Losses	(22)
Available-for-sale Investments, Total Fair Value	310,374
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Investments, Amortized Cost	199,811
Available-for-sale Investments, Gross Unrealized Gains	69
Available-for-sale Investments, Gross Unrealized Losses	(6)
Available-for-sale Investments, Total Fair Value	199,874
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Investments, Amortized Cost	26,694
Available-for-sale Investments, Gross Unrealized Losses	(7)
Available-for-sale Investments, Total Fair Value	26,687
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Investments, Amortized Cost	83,738
Available-for-sale Investments, Gross Unrealized Gains	84
Available-for-sale Investments, Gross Unrealized Losses	(9)
Available-for-sale Investments, Total Fair Value	$ 83,813

Fair Value Measurements and Available for Sale Investments - Schedule of Contractual Maturities of Available-for-sale Debt Securities (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Fair Value Disclosures [Abstract]
Due within one year	$ 261,450
One to two years	48,924
Total	$ 310,374

Convertible Notes Financing - Additional Information (Details) - Convertible Promissory Note - USD ($)	Jan. 04, 2018	Oct. 02, 2017
Debt Instrument [Line Items]
Amount of issued		$ 6,000,000
Interest rate		8.00%
Maturity date		Oct. 02, 2018
Amount to be raised by automatic conversion of the Note upon qualified equity financing		$ 40,000,000
Series A Convertible Preferred Stock
Debt Instrument [Line Items]
Number of shares issued for conversion of the Note	3,499,209

Asset Acquisitions and Contingent Consideration - Additional Information (Details) - USD ($)								3 Months Ended
	Sep. 30, 2018	Sep. 21, 2018	Jun. 30, 2018	Jan. 31, 2018	Jan. 04, 2018	Oct. 31, 2017	Oct. 02, 2017	Mar. 31, 2019	Mar. 31, 2018	Jun. 24, 2018
Business Acquisition [Line Items]
In process research and development								$ 1,000,000	$ 20,898,000
GB001
Business Acquisition [Line Items]
In process research and development									$ 19,148,000
Pulmokine, Inc. \| License Agreement \| GB002
Business Acquisition [Line Items]
Product license term							10 years
Milestones accrued								0
Pulmokine, Inc. \| License Agreement \| GB002 \| IPR&D
Business Acquisition [Line Items]
Upfront payment						$ 5,500,000
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
Aerpio Pharmaceuticals, Inc. \| License Agreement \| GB004 \| Maximum
Business Acquisition [Line Items]
Development and regulatory milestone payments, payable										$ 55,000,000
Commercial milestone payments, payable										85,000,000
Sales milestone payments, payable										$ 260,000,000
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
Adhaere Pharmaceuticals, Inc. \| GB1275 \| Maximum
Business Acquisition [Line Items]
Regulatory, development and sales milestones payable		$ 62,000,000

Asset Acquisitions and Contingent Consideration - Schedule of IPR&D Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Acquired Finite Lived Intangible Assets [Line Items]
Total in process research and development	$ 1,000	$ 20,898
GB001
Acquired Finite Lived Intangible Assets [Line Items]
Total in process research and development		19,148
Other Programs
Acquired Finite Lived Intangible Assets [Line Items]
Total in process research and development	$ 1,000	$ 1,750

Stockholders' Equity (Deficit) - 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, 2019	Mar. 31, 2018	Dec. 31, 2018
Stockholders Equity Deficit [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							5,862,450		7,482,032
Series A Convertible Preferred Stock \| Stock Restriction Agreements
Stockholders Equity Deficit [Line Items]
Shares subject to forfeiture						1,305,427
Founder
Stockholders Equity Deficit [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 Deficit [Line Items]
Conversion of convertible preferred stock into common stock, shares							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
Unrestricted shares of common stock were subject to service vesting conditions								4,580,444
Common Stock \| Stock Restriction Agreements
Stockholders Equity Deficit [Line Items]
Unrestricted shares of common stock were subject to service vesting conditions					4,580,444
Founder Shares
Stockholders Equity Deficit [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 Thousands		3 Months Ended
	Feb. 06, 2019	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares outstanding awarded		7,469,973		5,107,329
Stock-based compensation expense related to issuance of anti-dilution shares		$ 3,089	$ 605
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		$ 21.74
Unrecognized compensation costs		$ 55,700
Weighted-average period of cost expects to recognize		3 years 6 months
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average period of cost expects to recognize		3 years 9 months 18 days
Unrecognized compensation costs		$ 16,900
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		3,359,856
Shares outstanding awarded		2,390,144
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	700,000
Term of awards	10 years
Percentage of amount increase in outstanding shares	1.00%
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		6,285,478

Equity Incentive Plans - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Subject to Options Outstanding, as of December 31, 2018	5,107,329
Shares Subject to Options Outstanding, Options granted	2,390,144
Shares Subject to Options Outstanding, Option exercised	(27,500)
Shares Subject to Options Outstanding, Outstanding as of March 31, 2019	7,469,973	5,107,329
Shares Subject to Options Outstanding, Options vested and exercisable as of March 31, 2019	160,682
Weighted-Average Exercise Price, Outstanding as of December 31, 2018	$ 7.51
Weighted-Average Exercise Price, Options granted	21.74
Weighted-Average Exercise Price, Option exercised	2.61
Weighted-Average Exercise Price, Outstanding as of March 31, 2019	12.08	$ 7.51
Weighted-Average Exercise Price, Options vested and exercisable as of March 31, 2019	$ 3.20
Weighted-Average Remaining Contractual Life (Years), Outstanding as of December 31, 2018	9 years 7 months 6 days	9 years 8 months 12 days
Weighted-Average Remaining Contractual Life (Years), Options vested and exercisable as of March 31, 2019	8 years 10 months 24 days
Aggregate Intrinsic Value, Outstanding as of December 31, 2018	$ 72,708	$ 16,343
Aggregate Intrinsic Value, Options vested and exercisable as of March 31, 2019	$ 2,968

Equity Incentive Plans - Summary of Restricted Stock Activity (Details) - Restricted Stock	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Restricted Stock Units Outstanding, Nonvested at December 31, 2018 \| shares	7,482,032
Number of Restricted Stock Units Outstanding, Granted \| shares	0
Number of Restricted Stock Units Outstanding, Vested \| shares	(1,619,592)
Number of Restricted Stock Units Outstanding, Forfeited \| shares	0
Number of Restricted Stock Units Outstanding, Nonvested at March 31, 2019 \| shares	5,862,440
Weighted-Average Grant Date Fair Value, Nonvested at December 31, 2018 \| $ / shares	$ 4.01
Weighted-Average Grant Date Fair Value, Granted \| $ / shares	0
Weighted-Average Grant Date Fair Value, Vested \| $ / shares	4.31
Weighted-Average Grant Date Fair Value, Forfeited \| $ / shares	0
Weighted-Average Grant Date Fair Value, Nonvested at March 31, 2019 \| $ / shares	$ 3.92

Equity Incentive Plans - Stock-Based Compensation Expense Reported in Condensed Consolidated Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 3,089	$ 605
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	1,293	4
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 1,796	$ 601

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Non-cancelable operating sublease, expiration date for initial leased space	2024-12
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%
Lessee, operating lease, discount rate	7.00%
Weighted average remining lease term	5 years
Cash paid for operating lease liabilities	$ 1.2
Rent expense		$ 0.5

Commitments and Contingencies - Schedule of Lease Costs (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Leases [Abstract]
Operating lease cost	$ 753
Variable lease cost	395
Short-term lease cost	14
Total lease cost	$ 1,162

Commitments and Contingencies - Schedule of Gross Future Minimum Annual Rental Commitments (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Leases [Abstract]
2019 (remaining 9 months)	$ 2,216
2020	3,035
2021	3,123
2022	3,216
2023	1,690
2024	1,741
Total undiscounted rent payments	15,021
Present value discount	(2,314)
Present value	12,707
Current portion of operating lease liability (included as a component of Accrued expenses)	2,170
Noncurrent operating lease liabilities	10,537
Total operating lease liability	$ 12,707

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Leases [Abstract]
2019	$ 2,944
2020	3,035
2021	3,123
2022	3,216
2023	1,690
Thereafter	1,741
Total	$ 15,749

Subsequent Events - Additional Information (Details) - Subsequent Event - Term Loan	May 02, 2019 USD ($) Tranche
Subsequent Event [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%
Prepayment Occurs through First Anniversary of Closing Date
Subsequent Event [Line Items]
Percentage of prepayment fee	3.00%
First Anniversary of Closing Date through Second Anniversary of Closing Date
Subsequent Event [Line Items]
Percentage of prepayment fee	2.00%
Second Anniversary of Closing Date and Prior to May 1, 2024
Subsequent Event [Line Items]
Percentage of prepayment fee	1.00%
LIBOR
Subsequent Event [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%.
Tranche One
Subsequent Event [Line Items]
Remaining borrowing capacity	$ 40,000,000
Tranche Two
Subsequent Event [Line Items]
Remaining borrowing capacity	30,000,000
Tranche Three
Subsequent Event [Line Items]
Remaining borrowing capacity	$ 50,000,000

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) (Parenthetical) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Series A Convertible Preferred Stock
Issuance of shares, offering costs	$ 0.4