Inclusion Criteria:

          -  Subjects with histologically or cytologically confirmed metastatic or locally advanced
             unresectable gastric or GEJ adenocarcinoma positive for CLDN18.2.

          -  Subjects should not be eligible for curative surgery and should have been refractory
             to or have relapsed after two or more prior lines of standard systemic therapy that
             included a platinum, a fluoropyrimidine, either a taxane or irinotecan, and an
             approved vascular endothelial growth factor receptor (VEGFR) antibody/tyrosine kinase
             inhibitor (TKI).

          -  For subjects eligible for human epidermal growth factor receptor 2 (HER2) directed
             therapy, prior systemic therapy should have included a HER2 targeting antibody
             approved for treatment of gastric cancer.

          -  Subjects may also be included if the aforementioned therapeutic options were medically
             not appropriate for them. In these cases, the reason(s) why required prior therapies
             for gastric cancer were medically not appropriate should be documented in the
             subject's electronic case report form (eCRF).

          -  For dose-expansion only: Subjects with at least 1 measurable lesion greater than or
             equal to 10mm which has not undergone biopsy within 3 months of screening scan. This
             lesion cannot be biopsied at any time during the study.

        Exclusion Criteria:

          -  Any anticancer therapy or immunotherapy within 4 weeks of start of first dose.

          -  Untreated or symptomatic central nervous system (CNS) metastases, leptomeningeal
             disease, or spinal cord compression

          -  Autoimmune disorders requiring chronic systemic steroid therapy or any other form of
             immunosuppressive therapy while on study, eg, ulcerative colitis, Crohn's disease, or
             any other gastrointestinal autoimmune disorder causing chronic nausea, vomiting, or
             diarrhea. Recent or current use of inhaled steroids or physiological substitution in
             case of adrenal insufficiency is not exclusionary.

          -  Evidence or history within last 3 months of gastrointestinal inflammatory conditions
             not associated with the underlying cancer disease including gastrinomas, duodenitis,
             proven gastric ulcer, duodenal ulcer, pancreatitis, or subjects with recent gastric
             bleeding. Subjects may be included if the symptomatic/immunosuppressive treatment is
             discontinued more than 4 weeks prior to the first dose of AMG 910, symptoms have
             resolved, and gastroscopy does not indicate signs of active disease.