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) of ≥ 50 mL/minute calculated using the formula of Cockcroft and Gault (\[140 - Age\] × Mass \[kg\]/\[72 × Creatinine mg/dL\]) (x 0.85 if female). * Part 2 only: Calculated or measured creatinine clearance (CrCl) of ≥ 30 mL/minute calculated using the formula of Cockcroft and Gault (\[140 - Age\] × Mass \[kg\]/\[72 × Creatinine mg/dL\]) (x 0.85 if female). * INR or prothrombin time (PT) \< 1.5 × ULN except for participants receiving anticoagulation, who must be on a stable dose of anticoagulant therapy for 6 weeks prior to enrollment Additional Inclusion Criteria Part 2: * No prior treatment for metastatic or unresectable disease except for a maximum of 1 dose of chemotherapy with or without nivolumab; prior adjuvant, neo-adjuvant, and peri-operative therapy is allowed, provided it has been completed more than 6 months prior to the first dose of study treatment * Fibroblast growth factor receptor 2b (FGFR2b) ≥ 10% 2+/3+ tumor cells (TC) as determined by centrally performed immunohistochemistry (IHC) testing, based on tumor sample either archival (obtained within 6 months/180 days prior to signing pre-screening informed consent) or a fresh biopsy. Exclusion Criteria: * Prior treatment with any selective inhibitor of the fibroblast growth factor (FGF)-FGFR pathway * Known positive human epidermal growth factor receptor 2 (HER2) status * Untreated or symptomatic central nervous system disease metastases and leptomeningeal disease * Peripheral sensory neuropathy grade 2 or higher * Clinically significant cardiac disease * Other malignancy within the last 2 years (exceptions for definitively treated disease) * Chronic or systemic ophthalmologic disorders * Major surgery or other investigational study within 28 days prior to randomization * Palliative radiotherapy within 14 days prior to randomization * Abnormalities of the cornea that may pose an increased risk of developing a corneal ulcer * Active autoimmune disease that has required systemic treatment (except replacement therapy) within the past 2 years or any other diseases requiring immunosuppressive therapy while on study