A Trial of ONO-4578 Versus Placebo in Combination With Nivolumab and Chemotherapy in Participants With Gastric Cancer, Esophagogastric Junction Cancer, or Esophageal Adenocarcinoma

Key Inclusion Criteria: * Have advanced or metastatic gastric cancer, esophagogastric junction cancer, or esophageal adenocarcinoma that has been histologically confirmed to be predominant adenocarcinoma. * Have not been treated with systemic anticancer agents for advanced or metastatic gastric cancer, esophagogastric junction cancer, or esophageal adenocarcinoma. * Participants who have received neoadjuvant or adjuvant therapy may be included if no residual tumors are found after surgery (R0 resection) and if the therapy was completed at least 180 days before the date of recurrence. * Have measurable or nonmeasurable disease, as defined in Response Evaluation Criteria in Solid Tumors (RECIST) Guideline v1.1, and documented on computed tomography (CT) or magnetic resonance imaging (MRI) images within 42 days before Cycle 1 Day 1 (C1D1). * Tumor programmed cell death ligand 1 (PD-L1) expression must be confirmed as combined positive score (CPS ≥1) prior to randomization. * Participant must be able to provide tissues samples for analyses. Key Exclusion Criteria: * Participants with Human epidermal growth factor 2 (HER2)-positive or indeterminate gastric cancer, esophagogastric junction cancer, or esophageal adenocarcinoma as determined by local testing. Determination for HER2 positivity is made based on the reference in each site. If there is no reference, rough indication for positive is 3+ by immunohistochemistry (IHC), or 2+ by IHC and positive by in situ hybridization. * Participants whose tumors have known microsatellite instability-high or mismatch repair-deficient status. * Marked malnutrition requiring continuous enteral nutrition or intravenous hyperalimentation, or continuous infusion therapy with hospitalization. * Active autoimmune disease, or history of autoimmune disease that might recur, that may affect vital organ function, or that may require immune-suppressive treatment, including systemic corticosteroids. * Participants with vitiligo, type 1 diabetes mellitus, and Hashimoto thyroiditis on appropriate replacement therapy may be enrolled. Other protocol-defined inclusion and exclusion criteria apply.