Key Inclusion Criteria:

          -  Histologically-confirmed adenocarcinoma of the gastro-esophageal junction or stomach

          -  Male or female aged ≥ 18 years

          -  Negative HER2 status obtained from the most recent available tissue sample

          -  Inoperable recurrent, locally advanced adenocarcinoma or progressing stage IV
             adenocarcinoma of the gastro-esophageal junction or stomach, and disease progression
             after either standard first- or second-line treatment (Substudy 1), or after standard
             first-line treatment (Substudies 2 and 3)

          -  Positive test for eligible FGFR aberrations (FGFR2 fusions / rearrangements /
             amplifications; FGFR1, FGFR2, or FGFR3 mutations / short variants)

          -  For Substudies 1 and 3, measurable disease as defined by the Investigator using RECIST
             1.1 criteria

          -  ECOG PS of 0 or 1

          -  Men and women of childbearing potential must agree to avoid impregnating a partner or
             becoming pregnant, respectively, during the study, and for at least 150 days after the
             last dose of either investigational drug

        Key Exclusion Criteria:

          -  Prior anticancer or investigational drug treatment within an interval shorter than the
             following, as applicable:

               1. One chemotherapy or biological (e.g., antibody) cycle interval

               2. Five half-lives of any small molecule investigational or licensed medicinal
                  product

               3. Two weeks, for any investigational medicinal product with an unknown half-life

               4. Four weeks of curative radiotherapy

               5. Seven days of palliative radiotherapy

               6. 28 days of radiotherapy

          -  Prior treatment with FGFR Inhibitors (all substudies), and prior treatment with
             taxanes within 6 months prior to randomization and/or anti-VEGF(R) therapeutic
             antibody or pathway-targeting agents (Substudies 2 and 3), and prior treatment with
             anti-programmed cell death receptor-1 (PD-1) or anti-programmed death ligand-1 (PD-L1)
             therapeutic antibody or pathway-targeting agents (Substudy 3)

          -  Concurrent evidence of clinically significant corneal or retinal disorder

          -  History of clinically significant cardiac disorders and/or a QT interval corrected by
             Fridericia's formula (QTcF) > 450 ms for males or > 460 ms for females

          -  For Substudies 1 and 3, known CNS metastases

          -  Concurrent uncontrolled or active infection with human immunodeficiency virus (HIV;
             known HIV 1/2 antibodies positive); active hepatitis B virus (HBV) and hepatitis C
             virus (HCV) co-infection; active tuberculosis (for Substudies 2 and 3)

          -  Child-Pugh B or C liver cirrhosis, or a history of hepatic encephalopathy, hepatorenal
             syndrome, or clinically-meaningful ascites related to cirrhosis (for Substudies 2 and
             3)

          -  Administration of a live, attenuated vaccine within 30 days prior to randomization
             (for Substudy 3)

          -  Treatment with systemic corticosteroids (except for steroidal replacement therapy) or
             other systemic immunosuppressive medications within 2 weeks prior to first dose of
             study drug or anticipated requirement for systemic immunosuppressive medications
             during the study (for Substudy 3)