This study is for people in South Korea who have cancer in or around the stomach (gastric cancer) or cancer where the food pipe (esophagus) joins the stomach, called gastroesophageal junction (GEJ) cancer. Their cancer is locally advanced, unresectable, or metastatic. Locally advanced means the cancer has spread to tissue close by. Unresectable means the cancer cannot be removed by surgery. Metastatic means the cancer has spread to other parts of the body. In South Korea, VYLOY is approved for the treatment of gastric cancer or GEJ cancer. The people in this study will receive VYLOY as part of their usual treatment for their cancer. In standard clinical practice VYLOY is given to people slowly through a tube into a vein. The main aim of the study is to collect information in a real-world setting about the safety of VYLOY in people with gastric cancer or GEJ cancer in clinics in South Korea. This study will also help researchers learn how long people's gastric cancer or GEJ cancer stays stable. This study is about collecting information only. This is known as an observational study. The individual's doctor decides on treatment, not the sponsor (Astellas). The study will last about 1 year (54 weeks).