Pembrolizumab/Placebo Plus Trastuzumab Plus Chemotherapy in Human Epidermal Growth Factor Receptor 2 Positive (HER2+) Advanced Gastric or Gastroesophageal Junction (GEJ) Adenocarcinoma (MK-3475-811/KEYNOTE-811)

Inclusion Criteria: Inclusion criteria include, but are not limited to: * Histologically or cytologically confirmed diagnosis of previously untreated, locally advanced unresectable or metastatic human epidermal growth factor receptor 2 (HER2) positive gastric or gastroesophageal junction (GEJ) adenocarcinoma * HER2-positive defined as either immunohistochemistry (IHC) 3+ or IHC 2+ in combination with in-situ hybridization positive (ISH+) or fluorescent in-situ hybridization (FISH), as assessed by central review on primary or metastatic tumor * Has measurable disease as defined by Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST 1.1) as determined by the site investigator * Male participants must agree to use approved contraception * Female participants who are not pregnant or breastfeeding, and who are either not a woman of childbearing potential (WOCBP), or are a WOCBP who agrees to use approved contraception * Has a performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) Performance Scale within 3 days prior to the first dose of trial treatment * Has a life expectancy of greater than 6 months * Has adequate organ function Exclusion Criteria: Exclusion criteria include, but are not limited to: * Has had previous therapy for locally advanced unresectable or metastatic gastric/GEJ cancer * Has had major surgery, open biopsy or significant traumatic injury within 28 days prior to randomization, or anticipation of the need for major sur…