Inclusion Criteria:

          -  Histologically or cytologically confirmed diagnosis of locally-advanced unresectable
             or metastatic HER2+ gastric or gastroesophageal junction adenocarcinoma (GEC)

          -  HER2+ disease documented since progression of the most recent line of systemic
             therapy, as follows:

               -  Phase 2 paclitaxel dose optimization stage:

                    -  HER2 amplification in a blood-based NGS assay performed at a central
                       laboratory, or

                    -  HER2 overexpression/amplification immunohistochemistry (IHC) and in situ
                       hybridization (ISH) (IHC3+ or IHC2+/ISH+) assay of a tumor tissue sample

               -  Phase 2 dose expansion stage:

                    -  Cohort 2A: HER2 amplification in a blood-based NGS assay performed at a
                       central laboratory

                    -  Cohort 2B: No HER2 amplification by blood-based NGS assay, but HER2
                       overexpression/amplification by IHC and ISH (IHC3+ or IHC2+/ISH+) assay of a
                       tumor tissue sample

               -  Phase 3: HER2 amplification in a blood-based NGS assay performed at a central
                  laboratory

          -  History of prior treatment with a HER2-directed antibody

          -  Progressive disease during or after first-line therapy for locally-advanced
             unresectable or metastatic GEC

          -  Phase 2: Measurable disease according to RECIST version 1.1

          -  Phase 3: Measurable or non-measurable disease according to RECIST version 1.1

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

          -  Life expectancy of at least 3 months, in the opinion of the investigator

        Exclusion Criteria:

          -  Subjects with squamous cell or undifferentiated GEC

          -  Having received more than 1 line of prior systemic therapy for locally-advanced
             unresectable or metastatic disease

          -  Having received taxanes ≤12 months prior to enrollment, prior treatment with
             ramucirumab, or prior treatment with tucatinib, lapatinib, neratinib, afatinib, or any
             other investigational anti-HER2 and/or anti-EGFR tyrosine kinase inhibitor, or with
             T-DM1, T-Dxd, or any other HER2-directed antibody-drug conjugate

          -  Phase 2 paclitaxel dose optimization stage only: history of prior partial or total
             gastrectomy

          -  Unable to swallow pills