A Study to Learn About the Study Medicine Called PF-08634404 in Combination With Chemotherapy in Gastroesophageal Cancer

Inclusion Criteria: * Histological or cytological confirmed gastric, gastroesophageal junction or esophageal adenocarcinoma. * Evidence of locally advanced or metastatic disease. * Eastern Cooperative Oncology Group performance status (ECOG) 0-1 * No prior systemic therapy for advanced or metastatic disease. * Adequate hepatic, liver, and renal function * HER-2 negative status based on local testing * PD-L1 positive status based on local testing Exclusion Criteria: * Participants with known active CNS metastases, including leptomeningeal, brainstem, meningeal, or spinal cord metastases or compression * Clinically significant risk of hemorrhage or fistula * Major surgery or severe trauma within 4 weeks prior to the first dose, or planned major surgery during the study * History of allogeneic organ transplantation and allogeneic hematopoietic stem cell transplantation. * Any Grade ≥3 bleeding/hemorrhage events within 28 days of Cycle 1 Day 1, or prior history of clinically significant bleeding events * Clinically significant cardiovascular disease, or other comorbidities, within 6 months prior to first dose * Participants with active autoimmune diseases requiring systemic treatment within the past 2 years * Evidence of non-infectious or drug-induced interstitial lung disease (ILD) pneumonitis