Tucatinib, Trastuzumab, Ramucirumab, and Paclitaxel Versus Paclitaxel and Ramucirumab in Previously Treated HER2+ Gastroesophageal Cancer

This study is being done to see if tucatinib with trastuzumab, ramucirumab and paclitaxel works better than ramucirumab and paclitaxel to treat HER2-positive (HER2+) cancer of the gut (stomach or gastroesophageal cancer). This study will also look at what side effects happen when participants take this combination of drugs. A side effect is anything the drug does other than treating cancer. Study treatment will be given in 28-day cycles. In the Phase 2 part of the trial, participants and their doctors will know what drugs are being given (open-label). In the Phase 3 part, the study is "blinded." This means that participants, their doctor, and the study sponsor will not know which drugs are being given.