Key Inclusion Criteria:

          -  Histologically confirmed diagnosis of previously untreated locally advanced
             unresectable or metastatic HER2+ GC or GEJ adenocarcinoma

               1. Prior systemic perioperative treatment is allowed; however the patient must have
                  had a disease-free interval of at least 6 months from end of chemo/surgery

               2. Patients receiving perioperative anti-HER2 therapy require testing of HER2 status
                  for eligibility

               3. Cohort A: HER2-positive (by IHC 3+) and PD-L1-positive (by IHC with 22C3 CPS ≥
                  1%) per central review

               4. Cohort B: HER2-positive (by IHC 3+ or IHC 2+ in combination with FISH+) by local
                  review. PD -L1 status is not required for enrollment.

          -  Availability of formalin-fixed, paraffin-embedded tumor specimen, unstained slides or
             contemporaneous biopsy for tumor target testing

          -  Eastern Cooperative Oncology Group performance status of 0 or 1, verified within 3
             days of Day 1

          -  Life expectancy ≥ 6 months

          -  At least one radiographically measurable target lesion

          -  Acceptable laboratory parameters and adequate organ function

        Key Exclusion Criteria:

          -  Other malignancy that is progressing or required treatment within the past 5 years,
             with certain exceptions

               -  Patients with known MSI-H status

          -  History of allogeneic stem cell or tissue/solid organ transplant

          -  Central nervous system metastases

          -  Clinically significant cardiovascular disease, gastrointestinal disorders, pulmonary
             compromise

               -  Prior neoadjuvant or adjuvant treatment with immunotherapy