KQB198 in Combination With Imatinib in Participants With Advanced/Metastatic GIST in 1st Line Setting

This study will test an experimental drug called KQB198 in combination with imatinib. The goal is to determine if this combination is safe and tolerable and assess how effective the combination is at treating GIST. Imatinib has been approved by the FDA for the treatment of different types of cancer including GIST.

Inclusion Criteria All Participants: * Unresectable or metastatic disease * Tissue confirmation of GIST * Valid results from local testing of blood or tumor tissue documenting the presence of a KIT mutation (must not have exon 9 mutation) or PDGFRA mutation (must not have PDGFRA D842V). * Measurable disease per RECIST v1.1. * Patients must be in 1st line of treatment for advanced or metastatic disease. Prior imatinib is allowed in adjuvant or neoadjuvant setting, as long as imatinib was stopped over 1 year ago. * Adequate organ function and performance status Exclusion Criteria Participants are excluded from the study if any of the following criteria apply: * Unable to swallow or GI condition that prevents absorption. * Other active malignancies within the last 2 years. * History of hypersensitivity to any component of KQB198 or imatinib.