Inclusion Criteria:  Male or female ≥ 18 years of age Pathologically documented gastric or GEJ adenocarcinoma Disease progression on or after ≥ 2 prior platinum and fluoropyrimidine agents for advanced/metastatic disease ECOG PS 0-1 Willing and able to provide an adequate newly-acquired tumour sample for confirmation of HER2 status LVEF ≥ 50%  Exclusion Criteria:  Pleural effusion, ascites or pericardial effusion that requires drainage, peritoneal shunt, or Cell-free and CART. Drainage and CART. Spinal cord compression or brain metastases unless asymptomatic, stable, and not requiring steroids Active primary immunodeficiency, known HIV, active HBV, HCV infection. Uncontrolled infection requiring IV antibiotics, antivirals, or antifungals. History of non-infectious pneumonitis/ILD, current ILD, or where suspected ILD that cannot be ruled out by imaging at screening. Lung-specific intercurrent clinically significant severe illnesses.