Issuer Free Writing Prospectus

Filed pursuant to Rule 433

Registration File No. 333-237639

Relating to the

Preliminary Prospectus Supplements

dated May 18, 2020

(To Prospectus dated April 10, 2020)

PRICING TERM SHEET

May 18, 2020

Gossamer Bio, Inc.

Concurrent Offerings of

9,433,963 Shares of Common Stock, par value $0.0001 per Share

(the “Common Stock Offering”)

And

$200,000,000 Aggregate Principal Amount of

5.00% Convertible Senior Notes due 2027

(the “Convertible Notes Offering”)

The information in this pricing term sheet relates only to the Common Stock Offering and the Convertible Notes Offering (together, the “Offerings”) and supplements Gossamer Bio, Inc.’s (i) preliminary prospectus supplement, dated May 18, 2020 (the “Common Stock Preliminary Prospectus Supplement”) and (ii) preliminary prospectus supplement, dated May 18, 2020 (the “Convertible Notes Preliminary Prospectus Supplement” and, together with the Common Stock Preliminary Prospectus, the “Preliminary Prospectus Supplements”), and supersedes the information in the applicable Preliminary Prospectus Supplement to the extent inconsistent with the information in that Preliminary Prospectus Supplement. Neither the Common Stock Offering nor the Convertible Notes Offering is contingent on the completion of the other Offering. Terms used, but not defined, in this pricing term sheet have the respective meanings set forth in the applicable Preliminary Prospectus Supplement. As used in this pricing term sheet, “we,” “our” and “us” refer to Gossamer Bio, Inc. and not to its subsidiaries.

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If such make-whole fundamental change effective date or stock price is not set forth in the table above, then:

Notwithstanding anything to the contrary, in no event will the conversion rate be increased to an amount that exceeds 75.4716 shares of our Common Stock per $1,000 principal amount of Notes, which amount is subject to adjustment in the same manner as, and at the same time and for the same events for which, the conversion rate is required to be adjusted pursuant to the provisions described in the Convertible Notes Preliminary Prospectus Supplement under the caption “Description of Notes—Conversion Rights—Conversion Rate Adjustments—Generally.”

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We have filed a registration statement (including a prospectus) and the Preliminary Prospectus Supplements with the SEC for the offerings to which this communication relates. Before you invest, you should read the applicable Preliminary Prospectus Supplement and the prospectus in that registration statement and other documents we have filed with the SEC for more complete information about us and this offering. You may access these documents free by visiting EDGAR on the SEC website at www.sec.gov. Alternatively, we, any underwriter or any dealer participating in the Offerings will arrange to send you the applicable Preliminary Prospectus Supplement (or, when available, the applicable final prospectus supplement) upon request to: BofA Securities, Inc., NC1-004-03-43, 200 North College Street, 3rd floor, Charlotte, NC 28255-0001, Attention: Prospectus Department, or by email at dg.prospectus_requests@bofa.com or from SVB Leerink LLC, Attention: Syndicate Department, One Federal Street, 37th Floor, Boston, MA 02110, or by telephone at 1-800-808-7525, ext. 6218, or by email at syndicate@svbleerink.com; or, with respect to the Convertible Notes Offering, from Piper Sandler & Co., Attention: Prospectus Department, 800 Nicollet Mall, J12S03, Minneapolis, MN 55402, or by telephone at 1-800-747-3924 or by email at prospectus@psc.com.

The information in this pricing term sheet is not a complete description of the Common Stock, the Common Stock Offering, the Notes or the Convertible Notes Offering. You should rely only on the information contained or incorporated by reference in the applicable Preliminary Prospectus Supplement and the accompanying prospectus, as supplemented by this pricing term sheet, in making an investment decision with respect to the Common Stock and Notes.

Neither this pricing term sheet, the Common Stock Preliminary Prospectus Supplement nor the Convertible Notes Preliminary Prospectus Supplement and accompanying prospectuses constitute an offer to sell or a solicitation of an offer to buy any Common Stock or Notes in any jurisdiction where it is unlawful to do so, where the person making the offer is not qualified to do so or to any person who cannot legally be offered the Common Stock or Notes.

ANY DISCLAIMERS OR OTHER NOTICES THAT MAY APPEAR BELOW ARE NOT APPLICABLE TO THIS COMMUNICATION AND SHOULD BE DISREGARDED. SUCH DISCLAIMERS OR OTHER NOTICES WERE AUTOMATICALLY GENERATED AS A RESULT OF THIS COMMUNICATION BEING SENT VIA BLOOMBERG OR ANOTHER EMAIL SYSTEM.

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