• Age and Diagnosis:

⁃ Patients aged 18-75 years with gastric (G) or gastroesophageal junction (GEJ) adenocarcinoma, regardless of gender.

• Stage and Diagnosis Confirmation:

⁃ According to the 8th edition of AJCC staging for gastric cancer, patients must be assessed via abdominal CT as cStage III (cT3-4aN1-3M0).

⁃ Diagnosis must be confirmed by endoscopy and pathology as G/GEJ adenocarcinoma (HER-2 negative).

⁃ Only patients with Siewert type III and type II (who do not require combined thoracic surgery) GEJ cancer are eligible.

• Surgical Assessment:

⁃ Tumors must be deemed resectable with curative intent (R0 resection) as determined by a gastrointestinal surgeon and a radiologist.

⁃ Patients must agree to undergo radical surgery and be deemed operable by a surgeon.

• Previous Treatments:

⁃ Patients must not have received prior systemic treatment for the current disease, including surgery, anti-tumor chemotherapy, radiotherapy, or immunotherapy.

• Survival Expectancy:

⁃ Expected survival of at least 3 months.

• Measurable Disease:

⁃ Tumors must be measurable according to RECIST v1.1 criteria.

• Performance Status:

⁃ ECOG PS score of 0-1 within 7 days prior to the first dose.

• Cardiac Function:

⁃ Good cardiac function, suitable for curative surgery. Preoperative evaluation by a cardiologist is required if there are clinical indications of underlying ischemic, valvular, or other severe heart diseases.

• Organ Function:

⁃ Normal function of major organs, meeting the following laboratory criteria:

⁃ Absolute neutrophil count (ANC) ≥ 1.5 x 10\^9/L without granulocyte colony-stimulating factor for the past 14 days.

⁃ Platelet count ≥ 100 x 10\^9/L without transfusion for the past 14 days. Hemoglobin \> 9 g/dL without transfusion or erythropoietin use for the past 14 days.

⁃ Total bilirubin ≤ 1.5 x upper limit of normal (ULN); if \> 1.5 x ULN, direct bilirubin must be ≤ ULN.

⁃ AST and ALT ≤ 2.5 x ULN. Serum creatinine ≤ 1.5 x ULN and creatinine clearance (using Cockcroft-Gault formula) ≥ 60 ml/min.

⁃ Normal coagulation function (INR or PT ≤ 1.5 x ULN). Normal thyroid function (TSH within normal range, or normal T3/FT3 and FT4 if baseline TSH is out of range).

⁃ Normal myocardial enzyme spectrum (clinical judgment for non-significant lab abnormalities is acceptable).

• Weight:

⁃ Body weight ≥ 40 kg or BMI \> 18.5.

• Female Patients:

⁃ Must be post-menopausal (no menses for at least one year without other causes) or surgically sterilized (removal of ovaries and/or uterus).

⁃ Women of childbearing potential must have a negative pregnancy test within 7 days prior to the first dose.

⁃ Agree to use highly effective contraception (annual failure rate \< 1%) or abstain from heterosexual intercourse from the time of informed consent until at least 120 days after the last dose of the study drug or 9 months after surgery.

• Male Patients:

⁃ Must agree to use contraception or abstain from heterosexual intercourse from the time of informed consent until at least 120 days after the last dose of the study drug or 9 months after surgery.

⁃ Regular abstinence (e.g., calendar, ovulation, basal body temperature, or post-ovulation methods) and withdrawal are not acceptable forms of contraception.

• Informed Consent:

⁃ Patients must read, understand, and sign the informed consent form.