A Study to Investigate Tislelizumab Administered as Subcutaneous Injection Versus Intravenous Infusion Plus Chemotherapy in Patients With Unresectable or Metastatic Gastric or Gastroesophageal Junction Adenocarcinoma

Inclusion Criteria: * Histologically confirmed, locally advanced unresectable or metastatic gastric/ gastroesophageal junction (GEJ) adenocarcinoma. * No previous systemic therapy for locally advanced unresectable or metastatic gastric/GEJ cancer. * At least 1 measurable or nonmeasurable lesion per RECIST v1.1 as determined by investigator assessment. * Must be able to provide tumor tissues for biomarker assessment. * Eastern Cooperative Oncology Group (ECOG) Performance Status score ≤ 1. * Adequate organ function. * Women of childbearing potential must be willing to use a highly effective method of birth control for the duration of the study and ≥ 120 days after the last dose of tislelizumab. * Non-sterile males must be willing to use a highly effective method of birth control for the duration of the study and for ≥ 120 days after the last dose of tislelizumab. Exclusion Criteria: * Squamous cell or undifferentiated or other histological type gastric cancer (GC) * Active leptomeningeal disease or uncontrolled brain metastasis. Patients with equivocal findings or with confirmed brain metastases are eligible for enrollment provided that they are asymptomatic and radiologically stable without the need for corticosteroid treatment for ≥ 4 weeks before randomization. * Diagnosis with gastric or GEJ adenocarcinoma with positive human epidermal growth factor receptor 2 (HER2). * Active autoimmune diseases or history of autoimmune diseases that may relapse. * Uncontrollable pleural effusion, pericardial effusion, or ascites requiring frequent drainage (at least once a week) and/or diuretics within 7 days prior to randomization NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.