Efficacy Study of Nivolumab Plus Ipilimumab or Nivolumab Plus Chemotherapy Against Chemotherapy in Stomach Cancer or Stomach/Esophagus Junction Cancer

Inclusion Criteria: * Male or Female at least 18 years of age * Must have gastric cancer or gastroesophageal junction cancer that cannot be operated on and that is advanced or has spread out * Did not receive neoadjuvant or adjuvant treatment (chemotherapy, radiotherapy, or both) for their disease within the last 6 months * Must have full activity or, if limited, must be able to walk and carry out light activities such as light house work or office work * Must agree to provide tumor tissue sample, either from a previous surgery or biopsy within 6 months or fresh, prior to the start of treatment in this study Exclusion Criteria: * Presence of tumor cells in the brain or spinal cord that have not been treated * Active known or suspected autoimmune disease * Any serious or uncontrolled medical disorder or active infection * Known history of positive test for human immunodeficiency virus (HIV) or known acquired immunodeficiency syndrome (AIDS) * Any positive test result for hepatitis B or C indicating acute or chronic infection Other protocol-defined inclusion/exclusion criteria apply