The purpose of the study is to evaluate the efficacy and safety of postoperative adjuvant chemotherapy with PD-1 inhibitors and chemoradiotherapy, in comparison with adjuvant chemotherapy only, in D2/R0 resected pN3 gastric or gastroesophageal junction adenocarcinoma. PD-1+CRT cohort: A total of 216 patients will receive 6 weeks of PD-1 inhibitors and chemotherapy, then receive concurrent chemoradiotherapy, followed by 6 weeks of PD-1 inhibitors and chemotherapy, finally receive maintenance treatment of PD-1 inhibitors until (maximum 1year after radiotherapy). CT cohort: A total of 217 patients will receive 6 months of chemotherapy. The disease-free survival(DFS), overall survival(OS) and adverse effects will be analyzed.