Inclusion Criteria:

          1. Signed written informed consent.

          2. Age ≥ 18 years old (or minimum age based upon local regulations)

          3. Unresectable locally advanced or metastatic histologically proven HER2+
             gastroesophageal junction (GEJ) or gastric cancer. Gastric Cancer Expansion Phase will
             include only gastric cancer patients with 3+ HER2 positivity.

          4. HER2+ as 3+ (as defined in AJCC staging manual 8th edition) by IHC or in-situ
             hybridation (ISH) amplified.

          5. Have received prior treatment with trastuzumab.

          6. Have received treatment with at least one or more lines of cytotoxic chemotherapy in
             the metastatic setting.

          7. Resolution of chemotherapy, immunotherapy or radiation-related toxicities.

          8. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.

          9. Life expectancy ≥ 12 weeks.

         10. Measurable disease as per RECIST 1.1 criteria.

        Exclusion Criteria:

          1. Patients with symptomatic central nervous system (CNS) metastases.

          2. Patients with any history of known or suspected autoimmune disease with the specific
             exceptions of vitiligo, atopic dermatitis, or psoriasis not requiring systemic
             treatment.

          3. History of prior allogeneic bone marrow, stem-cell or solid organ transplantation.

          4. Treatment with any systemic anti-neoplastic therapy, or investigational therapy within
             the 3 weeks prior to the initiation of study drug.

          5. Treatment with radiation therapy within 3 weeks prior to the initiation of study drug
             administration.

          6. Treatment with corticosteroids (≥10 mg per day prednisone or equivalent) or other
             immune suppressive drugs within the 14 days prior to the initiation of study drug
             administration.

          7. History of clinically-significant cardiovascular disease.

          8. Clinically-significant pulmonary compromise, including a requirement for supplemental
             oxygen use to maintain adequate oxygenation.

          9. History of (non-infectious) pneumonitis that required steroids or presence of active
             pneumonitis

         10. Clinically-significant gastrointestinal disorders, such as perforation,
             gastrointestinal bleeding, or diverticulitis.

         11. Evidence of active viral, bacterial, or systemic fungal infection.