An Investigational Study of Immunotherapy Combinations With Chemotherapy in Patients With Gastric or Gastroesophageal Junction (GEJ) Cancers

Duration of Response (DOR)

Duration of Response (DOR) based on Blinded Independent Central Review (BICR) and investigator is defined as the time between the date of first documented complete response (CR) or partial response (PR) and the date of the first disease progression, per RECIST 1.1, or death due to any cause, or date of subsequent anticancer therapy, whichever occurs first. CR= Disappearance of all target lesions PR= At least a 30% decrease in the sum of diameters of target lesions

Time frame: From the date of first dose to the date of the first disease progression or death due to any cause, or date of subsequent anticancer therapy, whichever occurs first (Up to 63 months)

Overall Survival (OS)

Overall Survival (OS) is defined as the time between the date of randomization and the date of death due to any cause. For those without documentation of death, OS will be censored on the last date the participant was known to be alive.

Time frame: From the date of randomization to the date of death due to any cause (Up to 63 months)

Progression-Free Survival (PFS)

Progression-Free Survival (PFS) per Blinded Independent Central Review (BICR) and Investigator is defined as the time between the date of randomization and the first date of documented progression, or death due to any cause, or date of subsequent anticancer therapy, whichever occurs first. Participants who die without a reported prior progression (and die without start of subsequent therapy) will be considered to have progressed on the date of death. Progression=At least a 20% increase in the sum of diameters of target lesions, taking as reference the smallest sum on study and the sum must also demonstrate an absolute increase of at least 5 mm.

Time frame: From the date of randomization to the first date of documented progression, or death due to any cause, or date of subsequent anticancer therapy, whichever occurs first (Up to 63 months)

Number of Participants With Adverse Events (AEs) and Serious Adverse Events (SAEs)

Number of participants with any grade adverse events (AEs), serious adverse events (SAE), and adverse events leading to discontinuation using National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE v 5.0). An AE is defined as any new untoward medical occurrence or worsening of a preexisting medical condition in a clinical investigation participant administered study treatment and that does not necessarily have a causal relationship with this treatment. SAE is defined as any untoward medical occurrence that, at any dose results in death, is life threatening, requires inpatient hospitalization, results in significant disability, is a birth defect, or is an important medical event.

Time frame: From first dose to 30 days post last dose (Up to 60 months)

Number of Participants Who Died

Number of participants who died in each arm.

Time frame: Up to 60 months

Number of Participants With Laboratory Abnormalities in Specific Liver Tests

Number of participants with laboratory abnormalities in specific liver tests based on US conventional units. The number of participants with the following laboratory abnormalities from on-treatment evaluations will be summarized: * ALT or AST \> 3 x ULN, \> 5 x ULN, \> 10 x ULN and \> 20 x ULN * Total bilirubin \> 2 x ULN * ALP \> 1.5 x ULN * Concurrent (within 1 day) ALT or AST \> 3 x ULN and total bilirubin \> 1.5 x ULN * Concurrent (within 30 days) ALT or AST \> 3 x ULN and total bilirubin \> 1.5 x ULN * Concurrent (within 1 day) ALT or AST \> 3 x ULN and total bilirubin \> 2 x ULN * Concurrent (within 30 days) ALT or AST \> 3 x ULN and total bilirubin \> 2 x ULN

Time frame: From first dose to 30 days post last dose (Up to 60 months)

Number of Participants With Laboratory Abnormalities in Specific Thyroid Tests

Number of participants with laboratory abnormalities in specific thyroid tests based on US conventional units. The number of participants with the following laboratory abnormalities from on-treatment evaluations will be summarized: * TSH value \> ULN and * with baseline TSH value \<= ULN * with at least one FT3/FT4 test value \< LLN within 2-week window after the abnormal TSH test * with all FT3/FT4 test values \>= LLN within 2-week window after the abnormal TSH test * with FT3/FT4 missing within 2-week window after the abnormal TSH test. * TSH \< LLN and * with baseline TSH value \>= LLN * with at least one FT3/FT4 test value \> ULN within 2-week window after the abnormal TSH test * with all FT3/FT4 test values \<= ULN within 2-week window after the abnormal TSH test * with FT3/FT4 missing within 2-week window after the abnormal TSH test

Time frame: From first dose to 30 days post last dose (Up to 60 months)