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

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.