Inclusion Criteria:  Adults with unresectable, locally advanced or metastatic gastric or gastroesophageal junction cancer not amenable to curative therapy Confirmed fibroblast growth factor receptor 2b (FGFR2b) overexpression by immunohistochemistry (IHC) (central testing result) Eastern Cooperative Oncology Group (ECOG) less than or equal to 1 Measurable, evaluable, or non-evaluable disease as long as evaluable by Response Evaluation Criteria in Solid Tumors (RECIST) version 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 greater than or equal to 9 g/dl 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 greater than or equal to 30 mL/minute calculated using the formula of Cockcroft and Gault 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 or neo-adjuvant therapy for local disease is allowed if ended more than 6 months of 1st dose) 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 Abnormalities of the cornea that may pose an increased risk of developing a corneal ulcer