Inclusion Criteria Part 1 and Part 2:

- 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 has no contraindications to mFOLFOX6 chemotherapy or 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 without red blood cell (RBC) transfusion within 7 days prior
to the first dose of study treatment

- 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 or 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, neo-adjuvant, and
peri-operative therapy 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) ≥ 10% 2+/3+ tumor cells (TC) as
determined by centrally performed immunohistochemistry (IHC) testing, based on tumor
sample either archival (obtained within 6 months/180 days prior to signing
pre-screening informed consent) or a fresh biopsy.

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

Note: Other protocol defined Inclusion/Exclusion criteria may apply.

Eligibility last updated 5/2/23. Questions regarding updates should be directed to the study team contact.