This study investigates treatment of cadonilimab or placebo combined with SOX chemotherapy (oxaliplatin + tegafur-gimeracil-oteracil potassium) given before surgery (neoadjuvant) and cadonilimab or placebo combined with SOX chemotherapy after surgery (adjuvant), will work and be safe for the treatment of resectable (removable by surgery) gastric or gastroesophageal cancer.

This trial is a Phase 3 study. All patients are resectable gastric or gastroesophageal junction adenocarcinoma, Eastern Cooperative Oncology Group (ECOG) performance status 0-1.The purpose of this study is to evaluate the efficacy and safety of cadonilimab combined with chemotherapy for perioperative treatment of resectable gastric or gastroesophageal junction adenocarcinoma.

Inclusion Criteria:

1. Patients with histologically confirmed gastric or gastroesophageal junction adenocarcinoma with resectable disease (clinical stage T3-4aN+M0 or T4bNanyM0 per AJCC 8th edition).
2. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 at enrollment.
3. Has life expectancy of at least 6 months.
4. Availability of tumor sample prior to study entry.
5. Patients must undergo radical surgery.
6. Has adequate organ function.

Exclusion Criteria:

1. Patients with unresectable locally advanced disease or distant metastasis.
2. Histopathology or cytology confirmed other pathological types, such as adenosquamous cell carcinoma, squamous cell carcinoma, or GI stromal tumor.
3. Current or prior use of immunosuppressive medication within 14 days before randomization.
4. Has received prior anti-cancer therapy for the current malignancy.
5. Has an active infection requiring systemic therapy.
6. Contra-indication to any of the study drugs.
7. Has a known additional malignancy that is progressing or has required active treatment within the past 5 years.
8. Has an active autoimmune disease that has required systemic treatment in past 2 years.
9. Known active Hepatitis B or Hepatitis C virus infection.
10. Has had an allogenic tissue/solid organ transplant.