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 (\[140 - Age\]) × Mass \[kg\]/\[72 × Creatinine mg/dL\]) (x 0.85 if female)
  * international normalized ratio (INR) or prothrombin time (PT) less than 1.5 times ULN except for participants receiving anticoagulation, who must be on a stable dose of anticoagulant therapy for 6 weeks prior to enrollment

Exclusion Criteria:

* Prior treatment for metastatic or unresectable disease (Note: prior adjuvant, neo-adjuvant, and peri-operative therapy is allowed if completed more than 6 months prior to first dose of study treatment)
* Prior treatment with any selective inhibitor of fibroblast growth factor - fibroblast growth factor receptor (FGF-FGFR) pathway
* Known human epidermal growth factor receptor 2 (HER2) positive
* Untreated or symptomatic central nervous system (CNS) disease or brain metastases
* Peripheral sensory neuropathy greater than or equal to Grade 2
* Clinically significant cardiac disease
* Other malignancy within the last 2 years (exceptions for definitively treated disease)
* Chronic or systemic ophthalmological disorders
* Major surgery or other investigational study within 28 days prior to first dose of study treatment
* Palliative radiotherapy within 14 days prior to the first dose of study treatment
* Evidence of or recent history (within 6 months) of corneal defects, corneal ulcerations, keratitis, or keratoconus, history of corneal transplant, or other known abnormalities of the cornea that may pose an increased risk of developing a corneal ulcer.