Efficacy and Safety Evaluation of Sintilimab in Combination With IBI310 as Treatment in Patients With EBV-Positive Gastric Cancer

Inclusion Criteria: * Has histologically confirmed diagnosis of unresectable locally advanced,recurrent or metastatic gastric or GEJ malignant tumor (including squamous carcinoma, adenocarcinoma, Signet-ring cell carcinoma). * Confirmed EBV positive determined by in situ hybridization (ISH), analyzed with tumor tissue sample, either from a previous surgery or biopsy , within last 6 months * Male or Female at least 18 years of age * Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Has adequate organ function. * Expected survival\>=12 weeks * Women of childbearing potential (WOCBP) must have a negative urine or serum pregnancy test at the timing of enrollment. * Participants of childbearing potential must be willing to use an adequate method of contraception for the course of the study through 6 months after the last dose of study medication. Applied to Arms 1: Has histologically confirmed gastric/GEJ malignant tumor, and were regarded as having clinical stage T3-T4aN0M0 or T2～4aN+M0 Applied to Arms 2: Had no prior systemic treatment for metastatic disease. Applied to Arms 3: Received ≥1 prior systemic treatment for metastatic disease. Exclusion Criteria: * Has received prior therapy with an anti-programmed death (PD)-1, antiPD-L1, anti-PD L2, anti-CTLA-4 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor * Is currently participating in and receiving study therapy ,except those in the survival follow up period of an investigational agent study or non-interventional study . * Received systemic treatment with corticosteroids (\> 10 mg daily prednisone equivalent) or other immunosuppressive medications within 4 weeks of first dose. Inhaled or topical steroids ,adrenal replacement steroid doses and steroid of prevention allergic reaction of i.v. contrast agent are permitted in the absence of active autoimmune disease. * Received a live vaccine within 4 weeks of the first dose of study medication or plan to receive live vaccine during study period. * Has had major surgery (craniotomy, thoracotomy or laparotomy) within 4 weeks prior to first dose of study medication, or anticipation of the need for major surgery during the course of study treatment