A Phase III Open-label, Multicenter Trial of Maintenance Therapy With Avelumab (MSB0010718C) Versus Continuation of First-line Chemotherapy in Subjects With Unresectable, Locally Advanced or Metastatic, Adenocarcinoma of the Stomach, or of the Gastro-esophageal Junction

• 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