Avelumab in First-Line Maintenance Gastric Cancer (JAVELIN Gastric 100)

Inclusion Criteria: * Male or female participants greater than or equal to (\>=) 18 years * Disease must be measurable by Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST 1.1) * Participants with histologically confirmed unresectable locally advanced or metastatic adenocarcinoma of the stomach or gastro-esophageal junction (GEJ) * Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0 to 1 at trial entry * Estimated life expectancy of more than 12 weeks * Adequate haematological, hepatic and renal functions defined by the protocol * Negative blood pregnancy test at Screening for women of childbearing potential * Highly effective contraception for both male and female participants if the risk of conception exists * Other protocol defined inclusion criteria could apply Exclusion Criteria: * Prior therapy with any antibody or drug targeting T-cell coregulatory proteins * Concurrent anticancer treatment or immunosuppressive agents * Prior chemotherapy for unresectable locally advanced or metastatic adenocarcinoma of the stomach or gastro-esophageal junction (GEJ) * Tumor shown to be human epidermal growth factor 2 plus (HER2+) * Major surgery for any reason, except diagnostic biopsy, within 4 weeks of enrolment and/or if the participant has not fully recovered from the surgery within 4 weeks of enrolment * Participants receiving immunosuppressive agents (such as steroids) for any reason should be tapered off these drugs before initiation of the st…