Key Inclusion Criteria:

          -  Histologically documented gastric or gastroesophageal junctional adenocarcinoma (not
             amenable to curative therapy)

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1

          -  Adequate hematological, liver and kidney function. Measurable or non-measurable, but
             evaluable disease using RECIST v1.1

          -  FGFR2b overexpression as determined by a centrally performed IHC tissue test and/or
             FGFR2 gene amplification as determined by a centrally performed ctDNA blood based
             assay

          -  Candidate for mFOLFOX6 chemotherapy

        Key Exclusion Criteria:

          -  Untreated or symptomatic central nervous system (CNS) metastases

          -  Clinically significant cardiac disease,

          -  Peripheral sensory neuropathy >/= Common Terminology Criteria for Adverse Events
             (CTCAE) Grade 2

          -  Active infection requiring systemic treatment

          -  Known human immunodeficiency virus (HIV) or acquired immunodeficiency syndrome
             (AIDS)-related illness, or known active or chronic hepatitis B or C infection

          -  Prior treatment with any selective inhibitor of the fibroblast growth factor
             (FGF)-FGFR pathway

          -  Known abnormalities of the cornea that may pose an increased risk of developing a
             corneal ulcer

          -  Known positivity for HER2

          -  Women who are pregnant or breastfeeding

        Note: Other protocol defined Inclusion/Exclusion criteria may apply