Key Inclusion Criteria:  Histologically confirmed locally advanced, metastatic, and/or unresectable GIST Documented disease progression on or intolerance to imatinib Have at least 1 measurable lesion according to mRECIST v1.1 ECOG - 0 to 2 Have clinically acceptable local laboratory screening results (clinical chemistry and hematology) within certain limits  Key Exclusion Criteria:  Prior treatment with < 2 Tyrosine Kinase Inhibitors (TKIs) (Part 1b only) Prior treatment with any TKI other than imatinib (Part 2 only) Known PDGFR alpha D842V mutation or known succinate dehydrogenase deficiency Clinically significant cardiac disease Major surgeries (eg, abdominal laparotomy) within 4 weeks of the first dose of study drug Gastrointestinal abnormalities including, but not limited to, significant nausea and vomiting, malabsorption, external biliary shunt, or significant bowel resection that would preclude adequate absorption Any active bleeding excluding hemorrhoidal or gum bleeding Seropositive for HIV 1 or 2, or positive for hepatitis B surface antigen or hepatitis C virus (HCV) antibody. Active, uncontrolled, systemic bacterial, fungal, or viral infections at Screening Received strong CYP3A4 inhibitors or inducers Received sunitinib within 3 weeks (Part 1a, Part 1b)