Substudy 06D: Combination Therapies in Second Line (2L) Gastroesophageal Adenocarcinoma (MK-3475-06D/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 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 …