This is a phase II open-label study to evaluate the efficacy and safety of zanidatamab combined with chemotherapy as neoadjuvant/conversion therapy in patients with HER2-positive (IHC 3+ or IHC 2+) locally advanced or metastatic gastric/gastroesophageal junction adenocarcinoma. The study consists of two cohorts: a neoadjuvant cohort (Simon's two-stage design, n=46) for treatment-naive stage III locally advanced disease, and an exploratory conversion cohort for oligometastatic disease. Patients receive zanidatamab (30 mg/kg Q3W) plus oxaliplatin-based chemotherapy, with or without PD-1 inhibitor (tislelizumab or sintilimab). The primary endpoint is pathological complete response (pCR).