Bemarituzumab or Placebo Plus Chemotherapy in Gastric Cancers With Fibroblast Growth Factor Receptor 2b (FGFR2b) Overexpression

Inclusion Criteria: * Adults with histologically documented unresectable, locally advanced or metastatic gastric or gastroesophageal junction cancer not amenable to curative therapy * Fibroblast growth factor receptor 2b (FGFR2b) ≥10% 2+/3+ tumor cell staining 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 * Eastern Cooperative Oncology Group (ECOG) less than or equal to 1 * Measurable disease or non-measurable, but evaluable disease, according to Response Evaluation Criteria in Solid Tumors (RECIST) V 1.1 * Participant has no contraindications to mFOLFOX6 chemotherapy * Adequate organ and bone marrow function: * absolute neutrophil count greater than or equal to 1.5 times 10\^9/L * platelet count greater than or equal to 100 times 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 aminotransferase (AST) and alanine aminotransferase (ALT) less than 3 times the upper limit of normal (ULN) (or less than 5 times ULN if liver involvement). Total bilirubin less than 1.5 times ULN (or less than 2 times ULN if liver involvement); with the exception of participants with Gilbert's disease) * calculated or measured creatinine clearance (CrCl) of ≥ 30 mL/minute calculated using the formula of Cockcroft and Gault (\[140 - Age…