Bemarituzumab Plus Chemotherapy and Nivolumab Versus Chemotherapy and Nivolumab for FGFR2b Overexpressed Untreated Advanced Gastric and Gastroesophageal Junction Cancer.

Inclusion Criteria Part 1 and Part 2: * Adult with unresectable, locally advanced or metastatic (not amenable to curative therapy) histologically documented gastric or gastroesophageal junction adenocarcinoma * Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 * Measurable disease or non-measurable, but evaluable disease, according to Response Evaluation Criteria in Solid Tumours version 1.1 (RECIST v1.1) * Participant has no contraindications to nivolumab and either mFOLFOX6 or CAPOX chemotherapy as per local prescribing information. Participants in Part 1 must have no contraindications to mFOLFOX6. Participants in Part 2 with contraindications to mFOLFOX6 are permitted and may be administered the CAPOX regimen, if no contraindications for this regimen exist. Participants in Part 2 with contraindications to CAPOX are permitted and may be administered the mFOLFOX6 regimen, if no contraindications for this regimen exist * Adequate organ function as follows: * Absolute neutrophil count ≥ 1.5 x 10\^9/L * Platelet count ≥ 100 x 10\^9/L * Hemoglobin ≥ 9 g/dL without red blood cell (RBC) transfusion within 7 days prior to the first dose of study treatment * Aspartate aminotransaminase (AST) and Alanine aminotransaminase (ALT) \<3 x upper limit of normal (ULN) (or \< 5 x ULN if liver involvement) * Total bilirubin \<1.5 x ULN (or \< 2 x ULN if liver involvement or Gilbert's disease) * Part 1 only: Calculated or measured creatinine clearance (CrCl) …