This study is designed to assess the levels of drug exposure following treatment with tislelizumab administered as a subcutaneous (SC) injection compared to intravenous infusion (IV) as first-line therapy in adults with gastric or gastroesophageal junction (GEJ) that is locally advanced and cannot be surgically removed or has spread from the stomach to other areas of the body. Approximately 351 patients will be participating in this study. The study is composed of a screening period, a treatment period, and a follow-up period.

This is a double-blind, randomized, multiregion, comparative phase clinical study designed to evaluate the efficacy and safety of HLX22 in combination with trastuzumab and chemotherapy as first-line treatment in patients with HER2-positive locally advanced/metastatic adenocarcinoma of the gastric and/or gastroesophageal junction (G/GEJ).Eligible subjects will be randomized to the two groups based on a 1:1 ratio. Enrolled subjects shall be treated with the study drug until the loss of clinical benefit, death, intolerable toxicity, withdrawal of informed consent, or other reasons specified by the protocol (whichever occurs first).