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.