A Study of Cadonilimab Combined With Chemotherapy in People With Gastric or Gastroesophageal Junction Cancer

Inclusion Criteria: * Capable of providing signed informed consent. * Age ≥18 years at time of informed consent. * ECOG performance status (PS) of 0 or 1 at enrollment. * Pathologically confirmed gastric or GEJ adenocarcinoma with T1 N+ M0 or T2-4 N0-3 M0 (per AJCC 8th edition) resectable disease deemed eligible for radical surgery. A diagnostic laparoscopy is strongly recommended to confirm M0 status. * HER2-negative tumor (IHC 0, 1+, or IHC 2+ and FISH non-amplified). * Adequate organ and marrow function as defined below: 1. Hemoglobin ≥9.0 g/dL 2. Absolute neutrophil count ≥1.5 x 109/L 3. Platelet count ≥100 x 109/L 4. Serum bilirubin ≤1.5 x upper limit of normal (ULN). Patients with known Gilbert's disease who have bilirubin level ≤ 3 x ULN may be enrolled. 5. ALT and AST ≤2.5 x ULN 6. Albumin ≥2.8g/dL (albumin infusion is not allowed within 14 days before the start of treatment) 7. Measured creatinine clearance \>50 mL/min as determined by Cockcroft-Gault using actual body weight 7. Willing and able to undergo pre-treatment (during screening) and on-treatment tumor biopsies. 8\. Women of childbearing potential (as defined in Section 13.3) must have a negative serum pregnancy test within 72 hours prior to the first dose and agree to take effective contraception measures during the study drug administration and within 9 months after the last dose. 9\. Male patients with female partners of childbearing potential (as defined in Section 13.3) must agree to take effective contraception measures during the study drug administration and within 6 months after the last dose. Exclusion Criteria: * Histologies other than adenocarcinoma, including squamous/adenosquamous cell carcinoma, neuroendocrine, or GI stromal tumor. * HER2-positive tumor (IHC 2+ and FISH-amplified, or IHC 3+). * Prior anticancer treatment for the disease under study (chemotherapy, radiation therapy, chemoradiation, approved or investigational therapy) prior to initiation of study treatment. * Prior receipt of an anti-PD1 or anti-CTLA4 monoclonal antibody, including for prior non-gastric malignancy. * Underlying medical conditions that, in the Investigator's opinion, make the administration of cadonilimab hazardous, including but not limited to: 1. Interstitial lung disease (ILD), including history of ILD or non-infectious pneumonitis. 2. Active viral, bacterial, or fungal infections requiring parenteral treatment within 14 days of initiation of cadonilimab with the exception of prophylactic antibiotic treatment. 3. Clinically significant cardiovascular disease, such as New York Heart Association Class II or greater cardiac disease or cerebrovascular accident within 3 months prior to initiation of study treatment, myocardial infarction within 6 months prior to initiation of study treatment, or unstable arrhythmia. 4. A condition or unresolved AE from a prior investigational drug that may obscure interpretation of toxicity determination or AEs. 5. History of prior solid-organ transplant, including allogeneic bone marrow transplant. * Concurrent chronic medical condition requiring the use of supraphysiologic doses of corticosteroids (\>10 mg/day of oral prednisone or equivalent) or immunosuppressive medications (absorbable topical corticosteroids not excluded). * Any active autoimmune disease or documented history of autoimmune disease or syndrome that required systemic treatment in the past 2 years (i.e. with the use of disease-modifying agents, corticosteroids, or immunosuppressive drugs), except for vitiligo or resolved childhood asthma or atopy. 1. Replacement therapy (e.g. insulin, levothyroxine) is not considered a form of systemic treatment. 2. Participants with asthma who require intermittent use of bronchodilators, inhaled steroids, or local steroid injections not excluded. 3. Participants with hypothyroidism stable on hormone replacement or Sjogren's syndrome not excluded. * Known hypersensitivity to any excipient contained in the formulations of study interventions. * Prior active malignancy active within the previous 2 years except for locally curable cancers that have been apparently cured, such as basal or squamous cell skin cancer(s), superficial bladder cancer, or carcinoma in-situ of the cervix, breast, or prostate cancer. Other cases of prior malignanices may be allowed after discussion with the Principal Investigator. * Known HIV infection with detectable viral load. Patients on effective anti-retroviral therapy with undetectable viral load within 6 months are eligible for this trial. * Known acute hepatitis B or chronic hepatitis B infection with active untreated disease. For patients with evidence of chronic infection, the HBV viral load must be undetectable on suppressive therapy, if indicated. * Known hepatitis C infection with detectable viral RNA. Patients with HCV infection who are currently on treatment are eligible if they have an undetectable HCV viral load. * Receipt of a live attenuated vaccine within 30 days of study treatment initiation.