This clinical trial is testing a new treatment for patients with a type of stomach cancer called HER2-positive gastric cancer. The study is in its final stage, called Phase III, and is being conducted in many countries. It will compare three different treatment groups to see which works best and is safest. - The study may take place at up to 200-250 sites around the world. - You may join if you have HER2-positive gastric cancer and meet other health requirements. - Participation involves receiving a combination of cancer medicines, but there could be risks like side effects from these treatments. **HER2** is a protein that can promote the growth of cancer cells. **PD-L1 CPS** refers to a score that shows the presence of a protein that helps cancer hide from the immune system. Participants need a specific score to join. **Rilvegostomig** and **T-DXd** (Trastuzumab Deruxtecan) are new medicines being tested in this study. This study is not for people with certain health conditions, like uncontrolled infections or recent major surgeries.