Gossamer Bio, Inc.

Item 9B. Other Information.

During the three months ended December 31, 2025, none of our officers or directors adopted orterminated any contract, instruction or written plan for the purchase or sale of our securities that was intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) or any “non Rule 10b5-1 trading arrangement," except as described below:

•On October 31, 2025, Faheem Hasnain, CEO and President, adopted a Rule 10b5-1 trading arrangement providing for the sale of up to 1,125,000 shares of our common stock. The trading arrangement is intended to satisfy the affirmative defense in Rule 10b5-1(c). The duration of the trading arrangement is from March 15, 2026 until December 29, 2026, or earlier if and when all transactions under the trading arrangement are completed.

On March 16, 2026, we commenced a workforce reduction of 77 individuals, constituting approximately 48% of our workforce, to reduce our operating expenses. Our remaining management and employees will continue the development of seralutinib and explore potential regulatory paths forward. This workforce reduction is expected to be substantially completed by the end of May 2026.

In connection with the workforce reduction, on March 15, 2026, Richard Aranda, M.D., resigned from his position as the Chief Medical Officer of the Company effective as of April 15, 2026. The Company expects to execute a separation agreement with Dr. Aranda in connection with his departure, pursuant to which he will receive the severance provided under his employment agreement, in exchange for a general release of claims. Dr. Aranda will continue as a consultant to the Company to assist with transitional matters.

We currently estimate that we will incur charges associated with the workforce reduction of approximately $6.7 million primarily related to employee severance payments, benefits and related termination costs. We expect the majority of related charges to be recognized in the second and third quarters of 2026.

The estimates of the charges and expenditures that we expect to incur in connection with the workforce reduction, and the timing thereof, are subject to several assumptions and the actual amounts incurred may differ materially from these estimates. In addition, we may incur other charges or cash expenditures not currently contemplated due to unanticipated events that may occur, including in connection with the implementation of the workforce reduction.

Item 9C. Disclosure Regarding Foreign Jurisdictions that Prevent Inspections.

Not applicable.