Inclusion Criteria:

          1. HER2 positive advanced gastric cancer

          2. Be willing and able to provide written informed consent/assent for the trial.

          3. Be 19 years of age on day of signing informed consent.

          4. Have measurable disease based on RECIST 1.1.

          5. performance status of 0 or 1 on the Eastern Cooperative Oncology Group Performance
             Scale.

          6. Demonstrate adequate organ function

          7. Female subject of childbearing potential should have a negative urine or serum
             pregnancy or be willing to use birth control.

        Exclusion Criteria:

          1. Is currently participating and receiving study therapy or has participated in a study
             of an investigational agent and received study therapy or used an investigational
             device within 4 weeks of the first dose of treatment.

          2. Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any
             other form of immunosuppressive therapy within 7 days prior to the first dose of trial
             treatment.

          3. Has a known history of active Bacillus Tuberculosis

          4. Hypersensitivity to pembrolizumab or any of its excipients.

          5. Has had a prior anti-cancer monoclonal antibody within 4 weeks prior to study Day 1 or
             who has not recovered from adverse events due to agents administered more than 4 weeks
             earlier.

          6. Has had prior chemotherapy, targeted small molecule therapy, or radiation therapy
             within 2 weeks prior to study Day 1 or who has not recovered from adverse events due
             to a previously administered agent.

          7. Has a known additional malignancy that is progressing or requires active treatment
             within 3 years.

          8. Has known active central nervous system metastases and/or carcinomatous meningitis.

          9. Has active autoimmune disease that has required systemic treatment in the past 2 years

         10. Has known history of, or any evidence of active, non-infectious pneumonitis.

         11. Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent.

         12. Has known active Hepatitis B or Hepatitis C

         13. Has received a live vaccine within 30 days of planned start of study therapy.