Study of Adjuvant Chemotherapy With or Without PD-1 Inhibitors and Chemoradiotherapy in Resected pN3 Gastric (G) or GEJ Adenocarcinoma

Inclusion Criteria: * Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) score of 0 or 1 * Patients with expected survival time more than 6 months * Patients after standard D2/R0 resection * Postoperative histologically confirmed adenocarcinoma of the stomach or GEJ * Positive lymph nodes more than 7, stage pN3 * Patients without distant metastasis (M0) or M1 with abdominal exfoliated cell detection positive (CY1P0) * Patients' physical condition and visceral function allows following adjuvant therapy, including chemotherapy, chemoradiotherapy and PD-1 inhibitor therapy. * Patients' blood routine and biochemical indicators should meet the following standard: Hb≥90g/L, ANC≥1.5\*10\^9/L, PLT≥100\*10\^9/L, ALT \& AST≤2.5 U/L, TB ≤ 1.5 UNL, serum creatinine\<1 UNL. * Patients who are willing to obey regimens during the study. * Written informed consent is acquired before random entry, and patients should know that he/she has the right to quit, and following treatment won't be affected. * Patients are willing to provide samples of blood and tissue. Exclusion Criteria: * Patients with gross peritoneal metastasis (CY1P0 excluded) or distant metastasis. * Patients who has received any anti-tumor therapy before surgery. * Patients who had received radiotherapy for abdominal organs including stomach, liver, kidney, etc. * Patients who had active systematic autoimmune diseases which need systematic treatment within 2 years before first medication in the study, substitutive therapy (such as thyroxine, insulin, etc) excluded. * Patients diagnosed with immunodeficiency, or was receiving systematic glucocorticoid treatment or other immunosuppressive therapy within 7 days before medication, physiological dose of glucocorticoid is allowed (≤10 mg/d prednison or equivalent medication) * Patients who have known severe allergic reaction (≥level 3) to anti-PD-1 monoclonal antibody, 5-FU, Oxaliplatin or any auxiliary material. * Patient diagnosed with other malignant tumor in the past 5 years, excluding radical basal cell carcinoma of the skin and/or radical resected carcinoma in situ. * Patient with severe vital organ failure. * Pregnant or lactation period * Patient with known mental illness or drug abuse that may influence compliance. * Patient with known HIV infection, or active tuberculosis. * Untreated active hepatitis B * Patient with active HCV infection * Uncontrolled complications * Other situations that might disturb study results and compliance.