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) H…