RecruitingPhase 3Drug trialSHR-A1811 for Subjects With Her2-positive Gastric Cancer and Gastroesophageal Junction Adenocarcinoma After Progression on or After First-line Anti-HER2 Therapy-containing RegimenThis study will assess the efficacy and safety of SHR-A1811 compared with treatment chosen by the investigator in participants with HER2-positive (defined as immunohistochemistry \[IHC\] 3+ or IHC 2+/in situ hybridization \[ISH\]+) gastric or GEJ adenocarcinoma (based on \[American Society of Clinical Oncology (ASCO) College of American Pathologists (CAP) guidelines who have progressed on or after a first-line anti-HER2 therapy-containing regimen.