This is a first-in-human (FIH) study to evaluate the safety and tolerability and to determine the maximum tolerated dose (MTD) and/or the recommended Phase 2 dose (RP2D) of denikitug (also known as GS-1811) as monotherapy and in combination with zimberelimab in participants with advanced solid tumors. This study will be conducted in 6 parts (Parts A, B, and E: monotherapy, Parts C and D: combination therapy, and Part F for both monotherapy and combination therapy) in participants with advanced solid tumors who have received, been intolerant to, or been ineligible for all treatments known to confer clinical benefit or in participants with select solid tumors.