Perioperative HLX10 (Serplulimab) With S-1+Oxaliplatin (SOX) in Locally Advanced and PD-L1-Positive Gastric or Esophagogastric Junction Adenocarcinoma

Inclusion Criteria: * 1\) Histologically confirmed gastric adenocarcinoma or gastroesophageal junction adenocarcinoma at the study site. (For gastroesophageal junction adenocarcinoma, only patients classified as Siewert type II or type III who do not require surgery involving simultaneous thoracoabdominal incision are eligible.) * 2\) Tumor tissue determined to be PD-L1 positive (CPS \>= 5) based on pre-screening evaluation by the central laboratory. * 3\) Diagnosis of cT3-4N1-3M0 disease according to the AJCC/UICC-TNM 8th edition based on assessments performed within 28 days prior to randomization (the same day of the week 4 weeks before the randomization date is acceptable; the same applies hereafter), with no prior systemic treatment. Pre-randomization laparoscopy is not mandatory, but if performed, no non-curative factors (including liver metastasis, peritoneal metastasis, other distant metastases, or positive peritoneal cytology (CY1)) should be observed. * 4\) Judged by the investigator as capable of undergoing R0 resection. * 5\) Age between 18 years and 80 years on the day of enrollment. * 6\) ECOG performance status (PS) of 0 or 1. * 7\) The most recent laboratory values obtained within 14 days prior to randomization meet all of the following (however, transfusion, recombinant human thrombopoietin, and administration of granulocyte colony-stimulating factor (G-CSF) within 14 days prior to blood collection are not permitted): (1) Neutrophil count \>= 1500/mm\^3 (2) Hemoglobin \>= 9.0 g/dL (3) Platelet count \>= 10 x 10\^4/mm\^3 (4) Total bilirubin \<= 1.5 mg/dL (5) AST \<= 100 U/L (6) ALT \<= 100 U/L (7) Serum albumin \>= 3.0 g/dL (8) Serum creatinine \<= 1.5 mg/dL (9) APTT \<= 60 s (10) PT-INR \<= 1.5 * 8\) Females of childbearing potential should test negative for pregnancy test (serum or urine) within 7 days prior to randomization (the same day of the week is acceptable). * 9\) Male patients and females of childbearing potential agree to use contraception from the time of provision of informed consent through a specified period after the last administration of study treatment (until at least 120 days after the last dose of HLX10 and at least 180 days after the last dose of study drug other than HLX10 (S-1 or oxaliplatin)). * 10\) Written informed consent for study participation has been obtained from the patient. Exclusion Criteria: * 1\) Presence of another active malignancy within 5 years prior to enrollment or concurrently. Patients with cured, localized tumors such as basal cell carcinoma of the skin, squamous cell carcinoma of the skin, superficial bladder cancer, non-invasive prostate cancer, non-invasive cervical cancer, and non-invasive breast cancer are permitted. * 2\) Patients scheduled to undergo organ transplantation or bone marrow transplantation. * 3\) Myocardial infarction and/or uncontrolled arrhythmia (including QTc interval \>= 450 ms in males or \>= 470 ms in females) occurring within 6 months prior to randomization (QTc interval calculated using Fridericia's formula). * 4\) Cardiac dysfunction classified as NYHA Class III-IV, or left ventricular ejection fraction (LVEF) \< 50 percent on echocardiography. * 5\) Positive for any of the following: HIV antibody, HBs antigen, or HCV-RNA (HCV-RNA is measured only if HCV antibody is positive). * 6\) HBs antigen negative, HBs antibody and/or HBc antibody positive, and quantitative HBV-DNA detectable (patients are not excluded if HBV-DNA is below the limit of detection). * 7\) Active tuberculosis. * 8\) Interstitial pneumonia, pneumoconiosis, radiation pneumonitis, drug-related pneumonitis, and/or severe pulmonary dysfunction that may interfere with evaluation and management of suspected drug-related pulmonary toxicity. * 9\) Known active or suspected autoimmune disease, excluding patients whose disease is stable at randomization and who do not require systemic immunosuppressive therapy. * 10\) Patients who received live vaccination within 28 days prior to randomization, except for inactivated virus vaccines used for seasonal influenza prophylaxis. * 11\) Patients who require systemic corticosteroids (a therapeutic dose of \> 10 mg/day prednisone or equivalent) or other immunosuppressive medications within 14 days prior to randomization or during the study period. However, in the absence of active autoimmune disease, patients are permitted to use steroids at a dose of \<= 10 mg/day prednisone or equivalent, or inhaled steroids, or adrenal hormone replacement therapy. * 12\) Patients who have an active infection requiring systemic anti-infective treatment within 14 days prior to randomization, except for prophylactic antibiotic treatment (e.g., for prevention of urinary tract infection or chronic obstructive pulmonary disease). * 13\) Prior treatment with other antibody or drug therapies for immune checkpoint blockade, such as PD-1, PD-L1, or CTLA-4 therapy. * 14\) Patients currently receiving other clinical study treatment, or for whom the planned start of treatment in this study is \< 14 days from completion of the prior clinical study treatment. * 15\) History of severe hypersensitivity to the monoclonal antibody or any component of the study drug. * 16\) History of psychotropic drug abuse or drug dependence. * 17\) Patients with a disease that may increase the risk associated with study participation and use of the study drug, or with other severe acute or chronic diseases that, in the investigator's judgment, render the patient unsuitable for participation in the study.