**Zolbetuximab Study for Gastric Cancer** This study tests a new treatment for adults with certain types of stomach cancer and cancer where the esophagus meets the stomach, called gastroesophageal junction (GEJ) cancer. The treatment combines a drug called zolbetuximab with another drug called pembrolizumab and chemotherapy. Zolbetuximab works by making the body's immune system attack cancer cells. It's for cancers that don't have a certain protein called HER2, but do have a protein called Claudin 18.2. Participants will receive either the real treatment or a placebo, which looks like the drug but has no medicine. The study will last until the cancer worsens or the participant can't tolerate the treatment. The treatment will be given in cycles, which means it repeats every 6 weeks. **Key Points:** - Participants need to visit the clinic regularly for treatment and health checks. - Participants may receive the treatment until their cancer gets worse or they need another treatment. - Regular scans and health checks will be done to monitor the cancer's response to the treatment. People with certain health conditions or those on specific medications cannot join. The study aims to see if this treatment helps people live longer compared to current treatments.