Efficacy and Safety of Lenvatinib (E7080/MK-7902) Plus Pembrolizumab (MK-3475) Plus Chemotherapy in Participants With Advanced/Metastatic Gastroesophageal Adenocarcinoma (MK-7902-015/E7080-G000-321/LEAP-015)

Inclusion Criteria: * Has histologically and/or cytologically confirmed diagnosis of previously untreated, locally advanced unresectable or metastatic gastroesophageal adenocarcinoma * Is not expected to require tumor resection during the treatment course * Has gastroesophageal adenocarcinoma that is not human epidermal growth factor receptor 2 (HER-2)/neu positive * Has measurable disease as defined by Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 by scan with IV contrast as determined by the local site investigator * Male participants agree to refrain from donating sperm and agree to either remain abstinent from heterosexual intercourse as their preferred and usual lifestyle OR agree to use contraception, during the intervention period and for ≥7 days after last dose of lenvatinib or 90 days after last dose of chemotherapy-whichever comes last * Female participants not pregnant or breastfeeding are eligible to participate if not a women of childbearing potential (WOCBP), or if a WOCBP they either use a contraceptive method that is highly effective OR remain abstinent from heterosexual intercourse as their preferred and usual lifestyle, and do not donate eggs (ova, oocytes) to others or freeze/store for their own use, and abstain from breastfeeding during the intervention period through 120 days after last dose of pembrolizumab, 30 days after last dose of lenvatinib, or 180 days after last dose of chemotherapy-whichever occurs last * Has a performance status of …