Title: A Phase III, Randomized, Double-blind Trial Comparing Trastuzumab Plus Description: A clinical trial with patients who have Gastric or Gastroesophageal Cancer. Patients who might be able to join the study will need to have a sample of their cancer tissue (biopsy) tested to see if the cancer contains a protein called HER2 (human epidermal growth factor receptor 2). All the patients on study will receive chemotherapy and a drug called trastuzumab plus either pembrolizumab or placebo (not active drug). 4 Key Inclusion/Exclusion Criteria including Age Range If Applicable: Patients who are 18 years of age or older, both male and female are able to participate. Patients must have locally advanced or metastatic gastric or gastroesophageal cancer confirmed by biopsy and also HER2 positive. Patients cannot have had treatment for this stage of the cancer previously.