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

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

• Participants with gastroesophageal adenocarcinoma that is known to be human epidermal growth factor receptor 2 (HER2)/neu positive are excluded. Participants with unknown HER2 status are eligible.

• Has provided an archival tumor tissue sample or most recently obtained core, incisional, or excisional biopsy of a tumor lesion

• 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 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.