**Study Summary** This research is testing a new medicine called **DA-3501** for people with certain types of advanced cancer: stomach cancer (gastric), a type near the stomach called gastro-esophageal junction (GEJ) cancer, and pancreatic cancer. These cancers have a specific protein marker called **CLDN18.2**. The study is in its early stages, meaning it's the first time this medicine is being tested in humans. Participants will receive the medicine once every three weeks. The study will watch for side effects, check how the body processes the drug (pharmacokinetics or PK), and test for an immune response (anti-drug antibodies or ADA). **Key Points:** - **Duration & Visits:** The treatment is given every three weeks with regular assessments. - **Eligibility:** Must be at least 19 years old with specific cancer types and a life expectancy of at least 12 weeks. - **Exclusions:** Heart or stomach problems and certain infections, like hepatitis or HIV, may disqualify you. This is a clinical trial, so doctors will closely monitor the effects of the medicine to ensure safety and effectiveness. Participants will help researchers learn more about how this medicine works.