Description of the Business	9 Months Ended
Description of the Business	Sep. 30, 2020
Accounting Policies [Abstract]
Description of the Business	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, 2020, the Company had an accumulated deficit of $512.9 million. From the Company’s inception through September 30, 2020, the Company has funded its operations primarily through equity and debt financings. The Company raised $942.0 million from October 2017 through September 30, 2020 through Series A and Series B convertible preferred stock financings, a convertible note financing, its IPO, its Credit Facility (as defined in Note 5 below), and concurrent underwritten public offerings of its 5.00% convertible senior notes due 2027 (the “2027 Notes”) and common stock in May 2020. See Note 5 for additional information regarding the Credit Facility and the 2027 Notes. 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 and other capital sources, including potential collaborations, licenses and other similar arrangements. Management believes that it has sufficient working capital on hand to fund operations through at least the next twelve months from the date these condensed consolidated financial statements were available to be issued. There can be no assurance that the Company will be successful in acquiring additional funding, that the Company’s projections of its future working capital needs will prove accurate, or that any additional funding would be sufficient to continue operations in future years. COVID-19 The COVID-19 pandemic has caused significant business disruption around the globe. The extent of the impact of COVID-19 on the Company’s operational and financial performance will depend on certain developments, including the duration and spread of the pandemic and the impact on the Company’s clinical trials, employees and vendors. At this point, the degree to which COVID-19 may impact the Company’s financial condition or results of operations is uncertain. A prolonged pandemic could have a material and adverse impact on financial results and business operations of the Company, including the timing and ability of the Company to complete certain clinical trials and other efforts required to advance the development of its product candidates and raise additional capital. For example, the Company commenced enrolling patients for a Phase 1b clinical trial in pulmonary arterial hypertension ("PAH") in the first quarter of 2020, and because the Company temporarily paused enrollment as a result of the ongoing COVID-19 viral pandemic, the Company now expects to report topline results from this trial in the fourth quarter of 2020. In addition, due to the challenges of enrolling patients posed by the COVID-19 pandemic, the Company may experience delays in the commencement of its Phase 2 clinical trial of GB002 in PAH and enrollment of patients in its Phase 2 clinical trials of GB004 in ulcerative colitis and of GB002 in PAH, as well as delays in reporting data results from its ongoing trials.