Inclusion Criteria:

          1. Patients who are ≥ 18 years of age.

          2. Patients who have histologically confirmed metastatic or unresectable GIST.

          3. Patients who received imatinib and 1 or 2 other TKIs as prior treatment regimens.
             Patients who experienced intolerance to prior therapies must have objective disease
             progression prior to enrollment onto BLU-285-1303 study.

          4. Patients who have an Eastern Cooperative Oncology Group Performance Status (ECOG PS)
             of 0 to 1.

        Exclusion Criteria:

          1. Patients who have received prior treatment with avapritinib or regorafenib.

          2. Patients who have previously received more than 3 different TKI treatment regimens.

          3. Patients who are known to be both V-kit Hardy-Zuckerman 4 feline sarcoma viral
             oncogene homolog (KIT) and platelet-derived growth factor receptor alpha (PDGRFα) wild
             type.

          4. Patients who received any systemic anticancer therapy within 2 weeks before
             randomization.

          5. Patients with clinically significant cardiovascular disease, uncontrolled
             thromboembolic disease, or significant risk of bleeding.

          6. Patients who have a non-healing wound, ulcer, gastrointestinal perforation, or bone
             fracture.

          7. Patients who have poor organ function as defined by laboratory parameters specified in
             the protocol.

          8. Patients who have received neutrophil growth factor support within 14 days of
             randomization.

          9. Patients who require therapy with a concomitant medication that is a strong inhibitor
             or strong inducer of CYP3A4.

         10. Patients who have had a major surgical procedure within 14 days of randomization.
             Patient has significant traumatic injury within 28 days before randomization.

         11. Patients who have a history of another primary malignancy that has been diagnosed or
             required therapy within 3 years before randomization.

         12. Patients who have a history of a seizure disorder requiring anti-seizure medication.

         13. Patients who have metastases to the brain.

         14. Patients who have a QT interval corrected using Fridericia's formula (QTcF) of > 450
             msec.

         15. Women who are unwilling, if not postmenopausal or surgically sterile, to abstain from
             sexual intercourse or employ highly effective contraception from the time of
             randomization and for at least 30 days after the last dose of study drug. Men who are
             unwilling, if not surgically sterile, to abstain from sexual intercourse or employ
             highly effective contraception from the time of randomization and for at least 90 days
             after the last dose of study drug.

         16. Women who are pregnant.

         17. Women who are breastfeeding.

         18. Patients who have prior or ongoing clinically significant illness, medical condition,
             surgical history, physical finding, or laboratory abnormality as determined by the
             investigator.