Inclusion Criteria:  Patient has histologically confirmed metastatic or unresectable GIST as established by entry in a previous avapritinib clinical trial and has been receiving treatment with avapritinib on one of these trials. Patient continues to receive clinical benefit from avapritinib treatment, as assessed by the investigator. Patient or legal guardian, if permitted by local regulatory authorities, provides informed consent.  Exclusion Criteria:  Patient requires therapy with a concomitant medication that is a strong inhibitor or strong inducer of cytochrome P450 (CYP) 3A4 Patient has a history of intracranial bleeding either prior to or during avapritinib treatment Patients who have poor organ function, defined as Adverse Events of NCI CTCAE version 5.0 Grade 3 or higher at the time of enrollment must delay start of treatment until symptoms return to Grade 2 or baseline, or the start of treatment has been approved by the Sponsor. Patients who have ongoing cognitive or mood effects Adverse Events of NCI CTCAE version 5.0 higher than Grade 1 must delay start of treatment until symptoms return to Grade 1 or baseline. Women who are unwilling, if not postmenopausal or surgically sterile, to abstain from sexual intercourse or employ highly effective contraception from the time of enrollment and for at least 30 days after the last dose of avapritinib. Men who are unwilling, if not surgically sterile, to abstain from sexual intercourse or employ highly effective contraception from the time of first dose and for at least 90 days after the last dose of avapritinib. Women who are pregnant. Women who are breast feeding.