Key Inclusion Criteria:

1. Locally advanced unresectable or metastatic gastric cancer (GC) or gastroesophageal junction (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. Eastern Cooperative Oncology Group performance status (ECOG PS) ≤ 1 within 7 days prior to randomization
4. Adequate organ function ≤ 7 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-programmed cell death protein-1 (PD-1), anti-programmed cell death protein ligand-1 (PD-L1), anti-programmed cell death protein ligand-2 (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.