X0306 4 2019-02-12 0 0001728117 Gossamer Bio, Inc. GOSS 0001569248 Burow Kristina C/O ARCH VENTURE PARTNERS 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 1 0 0 0 Common Stock 2019-02-12 4 C 0 3682539 A 3682539 I See footnotes Common Stock 2019-02-12 4 C 0 345419 A 4027958 I See footnotes Common Stock 2019-02-12 4 C 0 3682539 A 3682539 I See footnotes Common Stock 2019-02-12 4 C 0 345419 A 4027958 I See footnotes Series A Preferred Stock 2019-02-12 4 C 0 16571429 D Common Stock 3682539 0 I See footnotes Series A Preferred Stock 2019-02-12 4 C 0 16571428 D Common Stock 3682539 0 I See footnotes Series B Preferred Stock 2019-02-12 4 C 0 1554388 D Common Stock 345419 0 I See footnotes Series B Preferred Stock 2019-02-12 4 C 0 1554388 D Common Stock 345419 0 I See footnotes The shares of the Issuer's Series A Preferred Stock and Series B Preferred Stock automatically converted into shares of the Issuer's Common Stock, for no additional consideration, at a ratio of 4.5 to one share, immediately prior to the consummation of the Issuer's initial public offering. The Series A Preferred Stock and Series B Preferred Stock had no expiration date. These shares are directly held by ARCH Venture Fund IX, L.P. ("ARCH IX"). ARCH Venture Partners IX, L.P. ("GPLP"), as the sole general partner of ARCH IX, may be deemed to beneficially own the shares held by ARCH IX. ARCH Venture Partners IX, LLC ("GPLLC"), as the sole general partner of GPLP, may be deemed to beneficially own the shares held by GPLP. The Reporting Person has an interest in the GPLP and the GPLLC but does not have voting or investment control over the shares held by ARCH IX. The Reporting Person disclaims beneficial ownership of the securities reported herein, except to the extent of her pecuniary interest therein. These shares are directly held by ARCH Venture Fund IX Overage, L.P. ("ARCH Overage"). ARCH Venture Partners IX Overage, L.P. ("Overage GPLP"), as the sole general partner of ARCH Overage, may be deemed to beneficially own the shares held by ARCH Overage. GPLLC, as the sole general partner of Overage GPLP, may be deemed to beneficially own the shares held by Overage GPLP. The Reporting Person has an interest in the Overage GPLP and the GPLLC but does not have voting or investment control over the shares held by ARCH Overage. /s/ Jeff Boerneke, Attorney-in-Fact 2019-02-12