Adjuvant Therapy for Potentially Immunotherapy-Responsive Gastric or Gastroesophageal Junction Adenocarcinoma

Inclusion Criteria: * The patient voluntarily joins this study and signs the informed consent form. * Age ≥18 years and ≤75 years, male or female. * Histologically confirmed gastric or gastroesophageal junction (GEJ) adenocarcinoma (predominantly adenocarcinoma) with stage II-III disease (TNM staging, 8th UICC/AJCC) who have not received preoperative neoadjuvant therapy. * ECOG performance status: 0-1. * Postoperative testing of gastric cancer specimens indicates: PD-L1 CPS ≥5, or EBV-positive, or dMMR. * No prior anti-tumor therapy for gastric cancer, including chemotherapy, targeted therapy, immunotherapy, or local radiotherapy. * Female patients of childbearing potential (not surgically sterilized) must use a medically acceptable contraceptive method (e.g., intrauterine device, contraceptive pills, or condoms) during the study treatment period and for 3 months after the end of the study treatment. Female patients of childbearing potential must have a negative serum or urine HCG test within 72 hours prior to study enrollment and must not be breastfeeding. Male patients must be surgically sterilized or agree to use an appropriate contraceptive method during the study period and for 3 months after the last dose of the investigational drug. * Baseline hematological and biochemical parameters must meet the following criteria: Hemoglobin ≥90 g/L; Absolute neutrophil count ≥1.5×10⁹/L; Platelet count ≥100×10⁹/L; Hepatic function: ALT ≤2.5×ULN, AST ≤2.5×ULN, TBIL ≤1.5×ULN (for patients with liver metastases: ALT ≤5×ULN, AST ≤5×ULN, TBIL ≤3×ULN); Renal function: Creatinine ≤1.5×ULN, or when creatinine \>1.5×ULN, endogenous creatinine clearance \>50 mL/min; Alkaline phosphatase ≤2.5×ULN; Thyroid-stimulating hormone (TSH) ≤1×ULN (if abnormal, T3 and T4 levels should be evaluated; patients may be enrolled if T3 and T4 levels are within normal limits). Exclusion Criteria: * The patient voluntarily joins this study and signs the informed consent form. (Note: This item appears to be an inclusion criterion and is likely misplaced. Please verify.) * Pregnant or breastfeeding women. * Women of childbearing potential with a positive baseline pregnancy test. * Distant metastasis diagnosed by CT/MRI/EUS. * Prior anti-tumor therapy, including chemotherapy, radiotherapy, or immunotherapy. * Diagnosis of another malignancy within the past 5 years (excluding basal cell or squamous cell carcinoma, superficial bladder cancer, carcinoma in situ of the cervix, or breast cancer). * Uncontrolled pleural effusion, pericardial effusion, or ascites. * Severe cardiovascular disease within 12 months prior to enrollment, such as symptomatic coronary artery disease, congestive heart failure ≥ Class II, uncontrolled arrhythmia, or myocardial infarction. * Concurrent upper gastrointestinal obstruction/bleeding, or digestive dysfunction/malabsorption syndrome that may affect the absorption of S-1. * Concurrent severe uncontrolled infection or other severe uncontrolled concomitant disease, moderate or severe renal impairment. * History of allergic reactions to any of the study drugs. * Use of corticosteroids or other systemic immunosuppressive therapy within 14 days prior to enrollment. * Receipt of an investigational drug within 4 weeks prior to enrollment (participation in another clinical trial). * Active autoimmune disease (including but not limited to: uveitis, enteritis, hepatitis, hypophysitis, nephritis, vasculitis, hyperthyroidism, hypothyroidism, and asthma requiring bronchodilator therapy). Subjects with hypothyroidism requiring only hormone replacement therapy, or skin conditions not requiring systemic treatment (e.g., vitiligo, psoriasis, or alopecia), may be enrolled. * History of primary immunodeficiency. * Use of immunosuppressive agents within 4 weeks prior to the first dose of study treatment, excluding nasal sprays, inhaled, or other topical corticosteroids, or physiological doses of systemic corticosteroids (i.e., not exceeding 10 mg/day of prednisone or equivalent), or prophylactic use of corticosteroids for contrast allergy. * Receipt of a live attenuated vaccine within 4 weeks prior to the first dose of study treatment, or planned receipt during the study period. * Known interstitial pneumonia or active pulmonary tuberculosis. * History of allogeneic organ transplantation or allogeneic hematopoietic stem cell transplantation. * Positive HIV antibody, active hepatitis B or hepatitis C (hepatitis B: HBsAg positive and HBV-DNA ≥10⁴ copies/mL; hepatitis C: HCV antibody and HCV-RNA positive, requiring concurrent antiviral therapy). * Other factors that, in the investigator's judgment, may affect subject safety or trial compliance, such as severe concomitant disease requiring treatment (including psychiatric disorders), severe laboratory abnormalities, or other family or social factors.