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 gastroesophagealjunction cancer not amendable to curative therapy.

• Ability to provide tumor sample, either archival (obtained within 6 months tojoining 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 ontumor sample provided.

• For Part 2, FGFR2b overexpression positive defined as FGFR2b ≥10% 2+/3+ TCdetermined 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 EvaluationCriteria 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 orneo-adjuvant therapy for local disease is allowed if ended more than 6 months of 1stdose).

• 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 treateddisease).

• Chronic or systemic ophthalmological disorders.

• Major surgery or other investigational study within 28 days of first study treatmentdose.

• Palliative radiotherapy within 14 days of first study treatment dose.

• Abnormalities of the cornea that may pose an increased risk of developing a cornealulcer.

• History or evidence of systemic disease or ophthalmological disorders requiringchronic use of ophthalmic corticosteroids.