Substudy 06C: A Study of Investigational Agents With Pembrolizumab (MK-3475) and Chemotherapy in Participants With First-Line Locally Advanced Unresectable/Metastatic Gastroesophageal Adenocarcinoma (MK-3475-06C/KEYMAKER-U06)

Inclusion Criteria: 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 first-line (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 blinded independent central review (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 * 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 h…