Tucatinib Plus Trastuzumab and Oxaliplatin-based Chemotherapy or Pembrolizumab-containing Combinations for HER2+ Gastrointestinal Cancers

This trial studies tucatinib to find out if it is safe when given with trastuzumab and other anti-cancer drugs (pembrolizumab, FOLFOX, and CAPOX). It will 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. It will also look at whether tucatinib works with these drugs to treat certain types of cancer. The participants in this trial have HER2-positive (HER2+) cancer in their gut, stomach, intestines, or gallbladder (gastrointestinal cancer).