⁃ 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

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

• Tumor tissue must be confirmed as negative for human epidermal growth factor receptor 2 (HER2) expression as classified by American Society of Clinical Oncology/College of American Pathologists (ASCO-CAP) guidelines

• Core/excisional biopsy of a tumor lesion not previously irradiated has been provided

• 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 within 3 days prior to the first dose of study intervention

• Has a life expectancy of at least 6 months