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

• Has histologically and/or cytologically confirmed diagnosis of previously untreated locally advanced unresectable or metastatic 1L gastroesophageal adenocarcinoma

• Has gastroesophageal adenocarcinoma that is known to be human epidermal growth factor receptor 2 (HER2)/neu-positive are excluded. HER2 status is not required if HER2/neu testing is not mandatory per local standard of care (SOC)

• Is not expected to require tumor resection during the treatment course

• Has not had prior systemic therapy administered in the recurrent or metastatic setting

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

• Participants who have adverse events (AEs) due to previous anticancer therapies must have recovered to \<Grade 1 or baseline. Participants with endocrine-related AEs who are adequately treated with hormone replacement therapy are eligible

• Has adequate organ function

• Has measurable disease per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) as determined by the local site investigator/radiology assessment and verified by blind independent review committee (BICR)

• Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 assessed within 3 days before allocation/randomization.

• Has a life expectancy of at least 6 months

• 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.

• Who has 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 (ART)