Inclusion Criteria Part 1:  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 must be a candidate to receive mFOLFOX6 and nivolumab  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 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); with the exception of participants with Gilbert's disease) Calculated or measured creatinine clearance (CrCl) of ≥ 50 mL/minute calculated using the formula of Cockcroft and Gault International Normalized Ratio (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 mFOLFOX6 with or without nivolumab. Prior adjuvant or neo-adjuvant therapy for localized disease 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) overexpression as determined by centrally performed immunohistochemistry (IHC) testing  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