Key Inclusion Criteria:

          1. Locally advanced unresectable or metastatic GC or GEJ carcinoma and have
             histologically confirmed adenocarcinoma

          2. No previous systemic therapy for locally advanced unresectable or metastatic
             gastric/GEJ cancer. NOTE: Participants may have received prior neoadjuvant or adjuvant
             therapy as long as it was completed and have no recurrence or disease progression for
             at least 6 months.

          3. ECOG PS ≤ 1 within 7 days prior to randomization

          4. Adequate organ function as indicated by the following laboratory values ≤ days prior
             to randomization

        Key Exclusion Criteria:

          1. Has squamous cell or undifferentiated or other histological type GC

          2. Active leptomeningeal disease or uncontrolled brain metastasis

          3. Diagnosed with gastric or GEJ adenocarcinoma with positive HER2

          4. Prior therapy with an anti-PD-1, anti-PD-L1, anti-PD-L2 or any other antibody or drug
             specifically targeting T-cell co-stimulation or checkpoint pathways

        NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.