Study of Pembrolizumab (MK-3475) Plus Chemotherapy Versus Placebo Plus Chemotherapy in Participants With Gastric or Gastroesophageal Junction (GEJ) Adenocarcinoma (MK-3475-585/KEYNOTE-585)

Inclusion Criteria: * Has previously untreated localized gastric or GEJ adenocarcinoma as defined by T3 or greater primary lesion or the presence of any positive nodes - N+ (clinical nodes) without evidence of metastatic disease. * Plans to proceed to surgery following pre-operative chemotherapy based on standard staging studies per local practice. * Is willing to provide tissue from a tumor lesion at baseline and at time of surgery. * Has an Eastern Cooperative Oncology Group (ECOG) performance status score of 0 to 1 within 3 days prior to the first dose of study treatment. * Has adequate organ function. * Male participants of childbearing potential must agree to use an adequate method of contraception for the course of the study through 180 days after the last dose of chemotherapy. * Female participants of childbearing potential must be willing to use an adequate method of contraception for the course of the study through 180 days after the last dose of chemotherapy or through 120 days after the last dose of pembrolizumab, whichever is greater. * Has life expectancy of greater than 6 months. Exclusion Criteria: * Has a history of (non-infectious) pneumonitis that required steroids or has current pneumonitis. * Has an active infection requiring systemic therapy. * Is currently participating in or has participated in a trial of an investigational agent or has used an investigational device within 4 weeks prior to the first dose of study treatment. * Has received prior ther…