Regorafenib in Patients With Metastatic and/or Unresectable Gastrointestinal Stromal Tumor

* In this research study, each planned "cycle" of the study lasts 4 weeks. In the first cycle, participants will come to the clinic on Days 1, 15 and 16. For cycles 2 through 4, they will come to the clinic on Days 1 and 15 of each cycle. For cycle 5 and beyond, they will come to the clinic on Day 1 of each cycle. Repeat tumor imaging will be performed at the end of every 2 cycles during study drug administration (e.g. end of cycles 2, 4, 6, etc.) * During each cycle, participants will take regorafenib by mouth, once a day in the morning, for 3 weeks followed by one week during which you do not take regorafenib (the "rest period"). * FDG-PET/CT (Positron Emission Tomography) scans are required as part of this study to monitor effects of the study drug on the participant's GIST. The first scan will take place before the first dose of study drug. If the first scan shows that the "tracer sugar" collection is increased in the participant's GIST, they will have up to 5 additional scans performed at different time points throughout their participation in this research study. * Participants may continue to participate in this research study for as long as they do not have serious side effects or their disease does not get worse.