Description of the Business	9 Months Ended
Sep. 30, 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. 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 September 30, 2019, the Company had an accumulated deficit of $279.5 million. From the Company’s inception through September 30, 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 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”), an agent and as a 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 September 30, 2019, 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 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. Subsequent Events The Company has evaluated subsequent events through November 12, 2019, the issuance date of the condensed consolidated financial statements, and has determined that there were no material subsequent events to recognize or disclose.

Description of the Business

9 Months Ended

Sep. 30, 2019

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 September 30, 2019, the Company had an accumulated deficit of $279.5 million. From the Company’s inception through September 30, 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 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”), an agent and as a 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 September 30, 2019, 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 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.

Subsequent Events

The Company has evaluated subsequent events through November 12, 2019, the issuance date of the condensed consolidated financial statements, and has determined that there were no material subsequent events to recognize or disclose.