Givastomig Combined With Nivolumab and Chemotherapy in Adults With CLDN18.2 Positive Metastatic Gastric Cancer (GIVA-2)

Inclusion Criteria: * Histologically confirmed unresectable, locally advanced, or metastatic gastric, gastroesophageal junction (GEJ), or esophageal adenocarcinoma (EAC). * Treatment-naïve for advanced/metastatic disease (prior adjuvant/neoadjuvant therapy allowed if ≥6 months since last dose). * CLDN18.2 positive (membrane intensity score ≥1+ on ≥1% of tumor cells). * PD-L1 positive (CPS ≥1). * At least 1 measurable lesion per RECIST v1.1. * ECOG performance status 0 or 1.Performance status * Adequate organ function, including: * Hematologic: WBC ≥2,000/μL; ANC ≥1,500/μL; platelets ≥100,000/μL; hemoglobin ≥9 g/dL * Hepatic: AST/ALT ≤3×ULN (≤5×ULN if liver metastases); bilirubin ≤1.5×ULN (≤3×ULN if Gilbert's)Hepatic * Renal: Creatinine ≤1.5×ULN or eGFR ≥50 mL/min/1.73 m²Renal * Life expectancy ≥90 days. * Women of childbearing potential (WOCBP) and men must use effective contraception during the study and for a defined period after treatment. * Willing and able to provide informed consent and comply with study procedures Exclusion Criteria: * HER2-positive tumors. * Second malignancy within 3 years, except certain skin or cervical cancers.Prior malignancy * Active or unstable gastrointestinal ulcer or bleeding within 6 weeks. * Active autoimmune disease requiring systemic therapy within past 2 years or ongoing immunosuppressive therapy. * Active pneumonitis or history requiring steroids/immunosuppressive therapy within 3 years. * Participation in another therapeutic clinical trial. * Major surgery or significant injury within 4 weeks prior to first dose, or planned major surgery within 6 months. * Radiotherapy within protocol-specified timeframes without adequate recovery. * Active CNS metastases or carcinomatous meningitis (previously treated brain metastases allowed if stable).Brain metastases * Significant cardiovascular disease (NYHA Class 3-4 CHF, recent MI, unstable angina, TIA/stroke, or major cardiac procedures within 6 months).Cardiac * Active or uncontrolled HIV, hepatitis B, or hepatitis C infection, or immunodeficiency (controlled infection allowed).HIV * Receipt of live vaccine within 30 days or other vaccines within 7 days of first dose. * Active infection requiring parenteral therapy. * Known hypersensitivity to study drug components (e.g., DPD deficiency). * Any other condition or laboratory abnormality that, in the investigator's judgment, increases risk or interferes with study participation.