A Study of the Combination of Oxaliplatin, Capecitabine, and Trastuzumab With Chemoradiotherapy in the Adjuvant Setting in Operated Participants With Human Epidermal Growth Factor Receptor-2 Positive (HER2+) Gastric or Gastroesophageal Junction Cancer

Inclusion Criteria: * Curatively resected HER2+ gastric or gastroesophageal junction adenocarcinoma; HER2+ status as defined by immunohistochemistry-2 positive or 3 positive with corroborative Fluorescence In Situ Hybridization+ result * Participants with stages between Stage IB (T1N1M0) and Stage IIIC * ECOG performance status score equal to or less than (\<=) 2 during screening * Left ventricular ejection fraction equal to or higher than (\>=) 55% with acceptable levels of liver and renal functions * No known contraindication to capecitabine, oxaliplatin, and trastuzumab * No contraindication for radiotherapy or has not received any previous radiotherapy to the gastric region for any reason Exclusion Criteria: * Participants with a malign condition in the last 5 years except squamous cell carcinoma of the skin * Previous neoadjuvant chemotherapy and/or radiotherapy * Any disruption in the physical integrity of the upper gastrointestinal tract (except surgical intervention for gastric or gastroesophageal junction carcinoma) * Known (previously diagnosed and ongoing) malabsorption syndrome * Active gastrointestinal bleeding * Participants with Stage IV gastric or gastroesophageal junction adenocarcinoma * Clinically significant cardiac or cardiovascular disease * Uncontrolled hypertension * Participants who have received any investigational anti-cancer treatment or are being treated in a concomitant investigational drug study * Abnormal laboratory values at screening for serum total bilirubin, alanine aminotransferase or aspartate amino transferase, alkaline phosphatase, absolute neutrophil count, platelet count, and/or hemoglobin * Known or suspected hypersensitivity against trastuzumab or proteins of rodents * Pregnancy or lactation