A Clinical Study of HLX10 Combined With Chemotherapy Versus Placebo Combined With Chemotherapy for Neoadjuvant/Adjuvant Treatment of Gastric Cancer

Inclusion Criteria: 1. Voluntary participation in the clinical study; fully understands and is informed of the study and has signed the Informed Consent Form (ICF); willing to comply with and able to complete all trial procedures. 2. The gender is not limited. When ICF is signed, the age is ≥ 18 years and ≤ 70 years old. 3. Histologically confirmed untreated gastric cancers, mainly adenocarcinoma. 4. Within 4 weeks prior to first dose, determined by the Independent Radiology Review Committee (IRRC) as: ≥ T3 and number of lymph node metastases ≥ 1 and no distant metastasis. 5. Prior to enrollment, the attending physician will evaluate to determine the eligibility for a R0 resection for the purpose of radical treatment. 6. Have good cardiac function and can be treated with radical resection. 7. tumor specimen testing results are PD-L1 positive (CPS ≥5). Subjects must provide the tumor tissues at screening or in the investigated surgery (if any), for PD-L1 expression level assessment. 8. Within 7 days before the first use of the study drug, ECOG: 0 \~ 1; 9. Expected survival 12 weeks; 10. The functions of the vital organs meet requirements. Exclusion Criteria: 1. Existence of other active malignant tumors within 5 years or at the same time. 2. Plan to perform or have undergone an organ or bone marrow transplant. 3. Myocardial infarction and poorly controlled arrhythmias occurred within 6 months prior to the first dose. 4. Existence of grade III - IV cardiac disorders defined by the NYHA or echocardiogram shows: LVEF (left ventricular ejection fraction) \< 50%. 5. Human immunodeficiency virus (HIV) infection. 6. Patients with active tuberculosis. 7. Patient with previous or current interstitial pneumonia, pneumoconiosis, radiation pneumonitis, drug-associated pneumonia, severely impaired lung function, etc. 8. Patients who have previously received other antibody/drug treatments for immune checkpoints, such as PD-1, PD-L1, and CTLA4 treatments. 9. Have diseases that may increase the risk of participating in the study and using the study medications, or other severe, acute, and chronic diseases and therefore are judged by the investigator to be unsuitable for clinical studies.