A Study to Evaluate Sacituzumab Tirumotecan (MK-2870) in Advanced/Metastatic Gastroesophageal Adenocarcinoma (MK-2870-015)

Inclusion Criteria: * Has a histologically or cytologically confirmed diagnosis of advanced, unresectable or metastatic gastric adenocarcinoma, gastroesophageal junction adenocarcinoma, or esophageal adenocarcinoma * Has measurable disease per Response Evaluation Criteria in Solid Tumors 1.1 (RECIST 1.1) as assessed by the local site investigator/radiology. Lesions situated in a previously irradiated area are considered measurable if progression has been shown in such lesions * Has received, and progressed on, at least 2 prior chemotherapy and/or immunotherapy regimens for advanced, unresectable or metastatic gastroesophageal adenocarcinoma * Participants are eligible regardless of human epidermal growth factor receptor-2 (HER2) status. Participants who are HER2+ must have previously received trastuzumab where available/appropriate * Has provided tumor tissue sample for determination of trophoblast cell-surface antigen 2 (TROP2) status by the central laboratory before randomization for stratification * Participants who have adverse events (AEs) due to previous anticancer therapies must have recovered to Grade ≤1 or baseline (except for alopecia and vitiligo). Participants with endocrine-related AEs who are adequately treated with hormone replacement therapy are eligible * Has adequate organ function * Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 3 days before randomization * Has ability to swallow oral medication for those who may receive…