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)

Pembrolizumab (200 mg) or placebo will be administered intravenously \[IV\] on day 1 of each 3-week cycle. Trastuzumab (8 mg/kg loading dose, 6 mg/kg maintenance dose) will be administered IV on day 1 of each 3-week cycle. SOC chemotherapy for the global cohort will either be FP (80 mg/m\^2 cisplatin administered IV on Day 1 of each 3-week cycle and 800 mg/m\^2 5-fluorouracil \[5-FU\] administered IV on Days 1-5 of each 3-week cycle) or CAPOX (1000 mg/m\^2 capecitabine administered orally twice daily \[BID\] on days 1-14 of each 3-week cycle and 130 mg/m\^2 oxaliplatin administered IV on Day 1 of each 3-week cycle). A Japan cohort will receive SOX chemotherapy consisting of S-1 (tegafur, 5-chloro-2,4-dihydroxypyridine \[CDHP\], and potassium oxonate \[Oxo\]) administered orally BID according to Body Surface Area (BSA) on Days 1-14 of each 3-week cycle and oxaliplatin (130 mg/m\^2) administered IV on Day 1 each 3-week cycle.