REACTION (Radiation Enhanced Assessment of Combination Therapies in Immuno-ONcology) - Nivolumab or Nivolumab in Combination With Other Immuno-oncology (IO) Agents After Targeted Systemic Radiation in Patients With Advanced Esophagogastric Cancer

Inclusion Criteria: * Men and women aged ≥ 18 years old. * Histologically proven (squamous cell or adenocarcinoma) esophageal or gastro-esophageal junction cancer or gastric cancer (core biopsy required) * Either a formalin fixed paraffin block or a minimum of ten 5-micron tissue section's (slides) of tumor biopsy sample must be available for biomarker evaluation. * Recurrent disease or Stage IV disease as per American Joint Committee on Cancer (AJCC) staging 8.0 - patients who decline systemic chemotherapy in the first line metastatic setting are eligible. * (Relatlimab arm only) LVEF assessment with documented left ventricular ejection fraction ( LVEF) \>/=50% by either echocardiogram TTE or multigated acquisition scan (MUGA) (TTE preferred test) within 6 months from first study drug administration,whichever is most recent. * Eastern Cooperative Oncology Group (ECOG) performance status 0-1 * Adequate organ function as follows: * Leukocytes ≥ 2,000/mm3 * Absolute neutrophil count (ANC) ≥ 1000/mm3 * Platelet count ≥ 100,000/mm3 * Hemoglobin ≥ 9 g/dL * Creatinine ≤ 2.0 mg/dL * Bilirubin (total) within normal institutional limits (except subjects with Gilbert Syndrome who must have total bilirubin \< 3.0 mg/dL) * Aspartate aminotransferase (AST) (SGOT), Alanine Aminotransferase (ALT) (SGPT), and alkaline phosphatase ≤ 2.5 times the institutional upper limit of normal * prothrombin time (PT) such that international normalized ratio (INR) is ≤ 1.5 (or an in-rang…