• Voluntarily agreed to participate in the study and signed an informed consent form;

• Age 18-75 years(both 18 and 75);

• Expected survival ≥ 12 weeks

• ECOG physical condition score of 0 or 1

• Locally advanced or metastatic gastric adenocarcinoma (including adenocarcinoma of the gastroesophageal junction) confirmed by histology and/or cytology

• Subjects will only have failed or been intolerant to prior standard first-line therapy (excluding paclitaxel), with no restriction on prior treatment with a PD-1/PD-L1 inhibitor.

• Confirmation of HER2 (IHC 1+, 2+, or 3+) and PD-L1 expression: for Phase II enrolled subjects, results of investigator-confirmed HER2 and PD-L1 expression will be accepted; for Phase III enrolled subjects, HER2 and PD-L1 expression will be accepted only as results from the central laboratory.

• Bone marrow function:

∙ Hemoglobin ≥ 9 g/dL (no blood transfusion and no erythropoietin treatment within 2 weeks prior to the examination);

‣ Absolute neutrophil count ≥ 1.5 × 109/L (must not receive granulocyte colony-stimulating factor treatment within 2 weeks prior to the examination)

‣ Platelet count ≥ 90 × 109/L (no platelet transfusion or treatment with recombinant human thrombopoietin within 2 weeks before the test);

• Liver function (based on normal values at the Clinical Trials Center):

∙ Serum total bilirubin ≤ 1.5 times the upper limit of normal (ULN);

‣ Alanine aminotransferase (ALT) and Mentholatum aminotransferase (AST) ≤ 2.5 × ULN in the absence of hepatic metastases; ALT and AST ≤ 5 × ULN in the presence of hepatic metastases;

⁃ Renal function (based on normal values at the Clinical Trials Center):

⁃ Blood creatinine ≤ 1.5 x ULN, or creatinine clearance (CrCl) ≥ 60 mL/min calculated by the Cockcroft-Gault formula method, or measured 24-hour urine CrCl ≥ 60 mL/min;

⁃ Coagulation:

• Prothrombin time (PT) ≤ 1.5 x ULN;

∙ Thrombin time (TT) ≤ 1.5 × ULN;

∙ Activated partial thromboplastin time (APTT) ≤ 1.5×ULN;

⁃ Cardiac function:

∙ New York Heart Association (NYHA) classification \<3;

‣ Left ventricular ejection fraction (LVEF) ≥ 50%;

⁃ The subject is able to provide specimens for central laboratory testing/review (at least 5 tissue sections) from the site of the primary or metastatic focus of the tumor within 3 years, preferably specimens taken after failure of first-line therapy;

⁃ Have at least one measurable lesion according to RECISTv1.1 criteria;

⁃ For female subjects: should be surgically sterilized, post-menopausal, or agree to use a medically approved contraceptive method (e.g., IUD, birth control pills, or condoms) for the duration of the study treatment and for 6 months after the end of the study treatment period; must have had a negative blood pregnancy test within 7 days prior to the study drug administration and must not be breastfeeding; and must not donate eggs for a period of 6 months from the time of signing of the informed consent form to the time of the last administration of the study drug. No egg donation for 6 months. For male subjects: should be surgically sterilized or agree to use a medically approved method of contraception during and for 6 months after the end of study treatment; no sperm donation from the time of signing the informed consent until at least 4 months after the last dose of study drug;

⁃ Be able to understand the requirements of the trial and be willing and able to comply with the trial and follow-up procedures.