Researchers want to learn if sacituzumab tirumotecan (MK-2870) alone or with other treatments can treat certain gastrointestinal (GI) cancers. The GI cancers being studied are either advanced (the cancer has spread to other parts of the body), or unresectable (the cancer cannot be removed with surgery). The goals of this study are to learn: * About the safety of sacituzumab tirumotecan alone or with other treatments and if people tolerate it * How many people have the cancer respond (get smaller or go away) to treatment