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.