Neoadjuvant and Adjuvant Therapy Studies of Sintilimab Combined With Chemotherapy With or Without Ipilimumab N01 in Resectable Gastric/Gastroesophageal Junction Adenocarcinoma

Inclusion Criteria 1. Signed written Informed Consent Form (ICF) and ability to comply with protocol-specified visits and related procedures. 2. Age ≥ 18 years. 3. Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1. 4. Expected survival ≥ 6 months. 5. Histologically confirmed adenocarcinoma of the stomach or gastroesophageal junction (GEJ). For GEJ cancer, only Siewert type III and Siewert type II participants not requiring combined thoracotomy are eligible. 6. Clinical stage T3-4Nany or TanyN+M0 (stage II-IVa) gastric/GEJ adenocarcinoma confirmed by endoscopic ultrasound or contrast-enhanced CT/MRI within 4 weeks before the first dose, per the American Joint Committee on Cancer (AJCC) 8th edition gastric cancer TNM staging system. 7. Within 4 weeks before the first dose, evaluated by a responsible surgeon based on medical history and confirmed to meet study requirements for radical R0 resection. Exclusion Criteria 1. Histologically or cytologically confirmed other pathologic types (e.g., squamous cell carcinoma, sarcoma, undifferentiated carcinoma) or combined gastrointestinal stromal tumor (GIST) before randomization. 2. Suspicious metastatic lesions or locally advanced unresectable disease, regardless of stage. 3. History of gastrointestinal perforation or fistula within 6 months before randomization. May be enrolled if perforation/fistula has been surgically treated (repaired/resected) and disease recovery/remission is confirmed by the investigator. 4. Active diverticulitis, intra-abdominal abscess, or gastrointestinal obstruction. 5. Inability to swallow, malabsorption syndrome, or uncontrolled nausea/vomiting/diarrhea, or other severe gastrointestinal diseases affecting drug intake/absorption. 6. Any life-threatening bleeding event within 3 months before randomization, or grade 3/4 gastrointestinal/variceal bleeding requiring endoscopic/surgical intervention. 7. Active uncontrolled bleeding or known bleeding diathesis.