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

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.