Inclusion Criteria:

• Histologically confirmed diagnosis of previously untreated locally advanced unresectable or metastatic HER2+ GC or GEJ adenocarcinoma:
  • Cohort A: HER2-positive (by IHC 3+) and PD-L1-positive (by IHC with 22C3 CPS ≥ 1%), and non-MSI-H per central review 1%), and non-MSI-H per central review;
  • 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.

Exclusion Criteria:

• Other malignancy that is progressing or required treatment within the past 5 years, with certain exceptions.
• Patients with MSI-H status by central test in Cohort A, or patients with known MSI-H status in Cohort B.
• 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.