This study wants to find a dose of ziftomenib that people with advanced gastrointestinal stromal tumors (CIST) can handle safely when taken with imatinib. We also want to learn how well these two drugs work together.
• To evaluate the safety and tolerability of DCC-3009 as monotherapy in participants with advanced GIST. • To determine the recommended dose(s) for expansion in Module A, and if reached, the maximum tolerated dose (MTD) of DCC-3009 as monotherapy.