RecruitingDrug trialKN026 in Combination With Chemotherapy in HER2 Positive Gastric Cancer Subjects Who Have Failed First-line TherapyKN026-001 is a two-stage study (Open-label stage/Randomized stage). Open-label stage is designed to evaluate the safety and efficacy of KN026 and chemotherapy when given together. Randomized stage is designed to evaluate the OS and PFS in patients receiving KN026 and chemotherapy compared to patients receiving placebo and chemotherapy.