The goal of the study is to learn about Safety and efficacy of preoperative adjuvant SOX regimen combined with nivolumab versus FLOT Regimen with nivolumab in HER2-negative Gastric or Gastroesophageal Junction Adenocarcinoma. The main question it aims to answer are:

• Safety and efficacy of preoperative adjuvant SOX regimen combined with nivolumab versus FLOT regimen with nivolumab for the treatment of HER2-negative Gastric or Gastroesophageal Junction Adenocarcinoma.
• Disease-free survival of preoperative adjuvant SOX plus nivolumab and FLOT plus nivolumab for HER2-negative Gastric or Gastroesophageal Junction Adenocarcinoma.

Participants will be divided into two groups to use a FLOT chemotherapy regimen plus nivolumab (one group) and a SOX chemotherapy regimen plus nivolumab (another group). Researchers would compare tumor regression grade, adverse effects and survival benefit of two neoadjuvant regimens.

Inclusion Criteria:

• 1. Histopathological confirmation of GC or GEJC.
• 2. Absence of prior anti-tumor treatments, encompassing surgical resection, chemotherapy, radiotherapy, or immunotherapy.
• 3. Age within the range of 18 to 75 years.
• 4. Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) score of 0-1.
• 5. Absence of concurrent malignancies.
• 6. For patients with resectable GC, those with locally advanced stage III and IVA were clearly included according to the 8th edition of the American Joint Committee on Cancer (AJCC) staging system, and there were no unresectable factors.
• 7. Patients with HER-2 negative.
• 8. Basal information such as hematology and pathological histology was complete.

Exclusion Criteria:

• 1. Refusal of surgical resection subsequent to neoadjuvant therapy.
• 2. Receipt of other ICIs during the study period.
• 3. Receipt of corticosteroids during the study period.
• 4. Have received any anti-tumor therapy such as chemotherapy, radiotherapy, immunotherapy, etc., or have been more than 180 days since the last treatment.
• 5. Confirmed recurrence of GC.
• 6. Hypersensitivity to the study medication.
• 7. Systemic medical conditions contraindicating chemotherapy.
• 8. Psychiatric illnesses contraindicating chemotherapy.
• 9. Acute infections necessitating antibiotic therapy.
• 10. Uncontrolled diabetes mellitus.
• 11. Metastatic disease.
• 12. Severe malnutrition.
• 13. Active autoimmune disorders.
• 14. Pregnancy or lactation.
• 15. Positive serological test for hepatitis B or C virus infection,
• 16. Untreated central nervous system metastases peripheral neuropathy.
• 17. Severe myelosuppression.
• 18. Severe hepatic or renal insufficiency (Child-Pugh C, estimated glomerular filtration rate [eGFR] <30 mL/min).
• 19. Significant cardiac history.
• 20. Patients with a history of allogeneic organ transplantation.
• 21. History of malignancy within the past 5 years (with the exception of curative, localized cancer).
• 22. Patients with multiple factors affecting oral medication.
• 23. Vaccination within 4 weeks prior to the first dose of study drug.
• 24. Patients who have received immune checkpoint inhibitors and develop serious adverse reactions after treatment and need to be permanently disabled.
• 25. The investigator believes that the subject has other serious systemic diseases or other reasons and is not suitable for this clinical study.

The patients came from several tertiary hospitals in Xi'an city, Shaanxi Province, and were diagnosed with locally advanced gastric cancer, and the patients would accept either the neoadjuvant SOX+nivolumab or FLOT+nivolumab regimen.