The main goal of this first-in-human (FIH) study is to learn about the safety and dosing of GS-2121 when given alone or in combination with zimberelimab (ZIM) in participants with advanced solid tumors. The primary objectives of this study are: * To assess the safety and tolerability of GS-2121 as monotherapy and GS-2121 in combination with zimberelimab in participants with advanced solid tumors. * To identify the maximum tolerated dose (MTD) / maximum administered dose (MAD) and/or the recommended phase 2 dose (RP2D) of GS-2121 as monotherapy and in combination with zimberelimab in participants with advanced solid tumors.