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 therapy with an anti-programmed cell death protein-1 (anti-PD-1), anti-programmed cell death-ligand 1 (anti-PD-L1), or anti-PD-L2 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (i.e., cytotoxic T-lymphocyte-associated protein 4 \[CTLA-4\], tumor necrosis factor receptor superfamily member 4 \[OX-40\], necrosis factor receptor superfamily member 9 \[CD137\]) or has previously participated in a MSD pembrolizumab (MK-3475) clinical study.
* Has received prior systemic anti-cancer therapy including investigational agents for the current malignancy.
* Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 14 days prior the first dose of study treatment.
* Has a known additional malignancy that is progressing or has required active treatment within the past 5 years. Note: Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or carcinoma in situ that have undergone potentially curative therapy are not excluded.
* Has a known severe hypersensitivity (≥ Grade 3) to pembrolizumab, its active substance and/or any of its excipients, or to any of the study chemotherapy agents and/or to any of their excipients.
* Has an active autoimmune disease that has required systemic treatment in past 2 years.
* Has a known history of human immunodeficiency virus (HIV) infection.
* Has a known history of Hepatitis B or known active Hepatitis C virus infection.
* Has a known history of active tuberculosis (TB).
* Female participants who are pregnant or breastfeeding or expecting to conceive children within the projected duration of the study, starting with the screening visit through180 days after the last dose of chemotherapy or through 120 days after the last dose of pembrolizumab, whichever is greater.
* Male participants who are expecting to father children within the projected duration of the study, starting with the screening visit through 180 days after the last dose of chemotherapy.
* Has had an allogenic tissue/solid organ transplant.
* Has received a live vaccine within 30 days prior to the first dose of study treatment.