⁃ The main inclusion criteria include but are not limited to the following:

• Has histologically and/or cytologically confirmed diagnosis of previously treated, second line (2L) (received first line (1L) treatment) gastric adenocarcinoma, gastroesophageal junction adenocarcinoma, or esophageal adenocarcinoma

• Has metastatic disease or locally advanced, unresectable disease

• Has experienced documented objective radiographic or clinical disease progression during or after 1L therapy containing any platinum/fluoropyrimidine doublet with or without immunotherapy

• Tumor tissue must be confirmed as negative for HER2 expression (IHC 0/1+ or IHC2+/in situ hybridization negative) as classified by American Society of Clinical Oncology/College of American Pathologists (ASCO-CAP) guidelines

• Can provide a core/excisional biopsy of a tumor lesion not previously irradiated (collected from a biopsy performed after the most recent systemic anticancer therapy regimen)

• AEs due to previous anticancer therapies must be ≤Grade 1 or baseline (except alopecia and vitiligo). Endocrine-related AEs adequately treated with hormone replacement are acceptable

• Has Eastern Cooperative Oncology Group performance status of 0 or 1

• Has a life expectancy of at least 3 months

• Participants who are hepatitis B surface antigen (HBsAg) positive are eligible if they have received Hepatitis B Virus (HBV) antiviral therapy for at least 4 weeks, and have undetectable HBV viral load prior to allocation/randomization

• Participants with history of Hepatitis C Virus (HCV) infection are eligible if HCV viral load is undetectable at screening

• Human Immunodeficiency Virus (HIV)-infected participants must have well controlled HIV on antiretroviral therapy