Pembrolizumab, Trastuzumab, HER2 Positive Gastric Cancer

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.