Inclusion Criteria:

1. Male or Female at least 18 years of age
2. Must have gastric cancer or gastroesophageal junction cancer that cannot be operated on and that is advanced or has spread out
3. Did not receive neoadjuvant or adjuvant treatment (chemotherapy, radiotherapy, or both) for their disease within the last 6 months
4. 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
5. 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:

1. Presence of tumor cells in the brain or spinal cord that have not been treated
2. Active known or suspected autoimmune disease
3. Any serious or uncontrolled medical disorder or active infection
4. Known history of positive test for human immunodeficiency virus (HIV) or known acquired immunodeficiency syndrome (AIDS)
5. Any positive test result for hepatitis B or C indicating acute or chronic infection

Other protocol-defined inclusion/exclusion criteria apply