A Study Evaluating Bemarituzumab in Combination With Other Anti-cancer Therapies in Subjects With Previously Untreated Advanced Gastric or Gastroesophageal Junction Cancer.

The main objectives of this study are to evaluate the safety and tolerability of bemarituzumab in combination with other anti-cancer therapies, and to evaluate the efficacy of bemarituzumab in combination with S-1 and oxaliplatin (SOX) and nivolumab as assessed by objective response.

Inclusion Criteria: * Adults with unresectable, locally advanced or metastatic gastric or gastroesophageal junction cancer not amendable to curative therapy. * Ability to provide tumor sample, either archival (obtained within 6 months to joining study) or fresh biopsy. * For certain arms for Part 1, FGFR2b overexpression positive defined as any FGFR2b 2+/3+ TC determined by centrally performed immunohistochemistry (IHC), based on tumor sample provided. * For Part 2, FGFR2b overexpression positive defined as FGFR2b ≥10% 2+/3+ TC determined by centrally performed IHC testing, based on tumor sample provided. * Easter Cooperative Oncology Group (ECOG) performance score less than or equal to 1. * Measurable or non-measurable disease as long as evaluable by Response Evaluation Criteria Solid Tumors (RECIST) version 1.1 * Participant has no contradictions to CAPOX/SOX plus or minus nivolumab. * Adequate organ function. * For Part 2, measurable disease according to RECIST v1.1. 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 of first study treatment dose. * Palliative radiotherapy within 14 days of first study treatment dose. * Abnormalities of the cornea that may pose an increased risk of developing a corneal ulcer. * History or evidence of systemic disease or ophthalmological disorders requiring chronic use of ophthalmic corticosteroids.