A Safety and Efficacy Study of Treatment Combinations With and Without Chemotherapy in Adult Participants With Advanced Upper Gastrointestinal Tract Malignancies

Key Inclusion Criteria: * Participants with histologically confirmed diagnosis of locally advanced unresectable or metastatic gastric, GEJ, or esophageal adenocarcinoma with life expectancy ≥3 months as assessed by the Investigator * Eastern cooperative oncology group (ECOG) Performance Score of 0-1 * At least one measurable target lesion per RECIST v1.1. * Adequate organ and marrow function * Able to provide an archival tumor sample that is representative of the cancer under investigation and suitable for central PD-L1 testing Key Exclusion Criteria: * Participants with underlying medical conditions that, in the Investigator's or Sponsor's opinion, will make the administration of investigational products hazardous * Only for Cohort A: Known Human Epidermal Growth Factor Receptor 2 (HER-2) positive tumor * Known untreated, symptomatic, or actively progressing central nervous system (brain) metastases. Participants with leptomeningeal metastases are excluded from enrollment. * Discontinued use of prior immune checkpoint therapy due to immune related adverse events; received prior treatment with an anti-TIGIT monoclonal antibody. * History of trauma or major surgery within 28 days prior to enrollment. Note: Other protocol defined Inclusion/Exclusion criteria may apply.