Adding Nivolumab to Usual Treatment for People With Advanced Stomach or Esophageal Cancer, PARAMUNE Trial

Inclusion Criteria: * Participants must have advanced or locally unresectable gastric, gastroesophageal junction or esophageal adenocarcinoma * Participants must have PD-L1 CPS (Combined Positive Score) ≥ 1. This test would have been performed as part of standard of care (SOC) pathology testing, using tissue obtained within two years prior to registration and collected prior to or after a frontline regimen * Participants must have a histologically confirmed diagnosis of microsatellite stable (MSS) and HER2 negative gastric, gastroesophageal junction, or esophageal adenocarcinoma * Participants must have documented unresectable and/or metastatic disease on CT or MRI imaging completed prior to registration. Imaging must have been completed within 28 days prior to registration for participants with measurable disease. CT scans or MRIs used to assess non-measurable disease must have been completed within 42 days prior to registration. All disease must be assessed and documented on the Baseline Tumor Assessment Form * Participants with treated brain metastases must have no evidence of progression on the follow-up brain imaging after central nervous system (CNS)-directed therapy. All treatment for brain metastases must have been completed at least 28 days prior to registration * Participants must have disease progression or intolerance to frontline standard of care (SOC) chemotherapy plus either nivolumab, pembrolizumab or any other PD-1 or PD-L1 inhibitor. Peri-operative chemothe…