Inclusion criteria:

          1. Male and female participants must be at least 18 years of age

          2. Disease Characteristics:

             Locally advanced, unresectable, or metastatic disease Pathologically documented
             adenocarcinoma of the stomach or GEJ with HER2 overexpression (IHC 3+ or ICH 2+/ISH+)

          3. For Part 1, progression on or after at least one prior trastuzumab containing Regimen.

             For Part 2, previously untreated for unresectable or metastatic adenocarcinoma of the
             stomach or GEJ with HER2 overexpression.

          4. Has measurable target disease assessed by the Investigator based on RECIST version 1.1

          5. Has protocol defined adequate organ function including cardiac, renal and hepatic
             function

          6. If of reproductive potential, agrees to use a highly effective form of contraception
             or avoid intercourse during and upon completion of the study.

        Exclusion criteria:

          1. History of active primary immunodeficiency, known HIV, active HBV or HCV infection.

          2. Uncontrolled intercurrent illness.

          3. History of non-infectious pneumonitis/ILD, current ILD, or where suspected ILD that
             cannot be ruled out by imaging at screening.

          4. Lung-specific intercurrent clinically significant severe illnesses.

          5. Uncontrolled infection requiring intravenous (IV) antibiotics, antivirals, or
             antifungals.

          6. Pleural effusion, ascites or pericardial effusion that requires drainage, peritoneal
             shunt, or Cell-free and Concentrated Ascites Reinfusion Therapy (CART).

          7. Has spinal cord compression or clinically active central nervous system metastases.