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

The main objective of Part 1 is to evaluate the safety and tolerability of bemarituzumab plus 5-fluorouracil, leucovorin, and oxaliplatin (mFOLFOX6) and nivolumab. The main objective Part 2 is to compare efficacy of bemarituzumab plus chemotherapy (mFOLFOX6 or capecitabine combined with oxaliplatin (CAPOX)) and nivolumab to placebo plus chemotherapy (mFOLFOX6 or CAPOX) and nivolumab as assessed by overall survival.

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) \