A Study of KN026-based Combination Therapy in HER2-positive Gastric Cancer

Inclusion Criteria: Age≥18 years old. * Histologically or cytologically confirmed diagnosis of gastric cancer. * Participants unresectable locally advanced or metastatic gastric cancer who had not received systemic treatment (participants who had progressed 6 months after prior neoadjuvant/adjuvant therapy could be enrolled). * Confirmed to be HER2 positive (HER2-positive is defined as IHC 3+ or IHC 2+ with ISH positive) and PD-L1 status (participants of phase III should be confirmed by the pathology department of participating study center). * Phase II: Presence of at least 1 measurable lesion per RECIST 1.1. Tumor lesions situated in a previously irradiated area are considered measurable if progression has been demonstrated in such lesions. Phase III:Presence of at least 1 evaluable lesion per RECIST 1.1. Tumor lesions situated in a previously irradiated area are considered measurable if progression has been demonstrated in such lesions. * ECOG PS of 0 - 1. * Expected survival ≥ 3 months. * Participants with adequate organ functions. * Female and male participants of childbearing age agree to take adequate contraceptive measures during and upon completion of the study for 7 months after the last dose. Female participants of childbearing age must have a negative blood pregnancy test within 7 days before randomization. * Voluntarily agree to participate in the study and sign the informed consent. Exclusion Criteria: * Has received anti-tumor treatment such as systemic che…