This is a phase 1/2, multicenter, open-label umbrella platform study that will evaluate the safety and tolerability of investigational agents with pembrolizumab and fluoropyrimidine chemotherapy for the first-line (1L) treatment of participants with locally advanced unresectable or metastatic human epidermal growth factor receptor 2 (HER2)-negative gastric, gastroesophageal junction, or esophageal adenocarcinoma. This substudy will have two phases: a safety lead-in phase and an efficacy phase. The safety lead-in phase will be used to evaluate the safety and tolerability, and to establish a recommended Phase 2 dose (RP2D) for investigational agents in combination with chemotherapy and immunotherapy. There is no formal hypothesis in this study.