Tislelizumab in Combination With Pre-operative CRT Versus SOC for Locally Advanced G/GEJ Adenocarcinoma

Inclusion Criteria: 1. Has previously untreated localized gastric or GEJ adenocarcinoma as defined by T3\~4aN+M0 or T4bNanyM0 （AJCC Version 8） 2. Has an Eastern Cooperative Oncology Group (ECOG) performance status score of 0 to 1 3. Has adequate organ function. 4. Male participants of childbearing potential must agree to use an adequate method of contraception for the course of the study through 180 days after the last dose of chemotherapy. 5. Female participants of childbearing potential must be willing to use an adequate method of contraception for the course of the study through 180 days after the last dose of chemotherapy or through 120 days after the last dose of pembrolizumab, whichever is greater. Exclusion Criteria: 1. Has a known additional malignancy that is progressing or has required active treatment within the past 5 years. Note: Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or carcinoma in situ that have undergone potentially curative therapy are not excluded. 2. Has received prior systemic anti-cancer therapy including investigational agents for the current malignancy. 3. Has an active infection requiring systemic therapy. 4. Has a history of (non-infectious) pneumonitis that required steroids or has current pneumonitis. 5. Is currently participating in or has participated in a trial of an investigational agent or has used an investigational device within 4 weeks prior to the first dose of study treatment. 6. Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 14 days prior the first dose of study treatment. 7. Has an active autoimmune disease that has required systemic treatment in past 2 years. 8. Has a known history of human immunodeficiency virus (HIV) infection. 9. Has a known history of Hepatitis B or known active Hepatitis C virus infection (HBsAg positive with HBV DNA≥500 IU/ml；HCV：HCV antigen positive with HCV copies \>ULN). 10. Has had an allogenic tissue/solid organ transplant. 11. Has received a live vaccine within 30 days prior to the first dose of study treatment. 12. Female participants who are breastfeeding.