Symbiotic-GI-16: 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