Hi Sam and welcome.
Bruce and Katie *blows kiss to Katie* are most certainly right. Please let me see if I can also help clarify.
Both, GK and CK are FDA approved, thus, insurance companies do cover these treatment plans.  The key is... how much do they cover is all based on your individual insurance coverage.
I have BCBS of MA, had my CK at Beth Israel (an acute care facility).  Based on my plan, which is PPO, the entire process was covered 100% (for me) except for my dr office co-pays and Rx.  When you call BCBS, they will confirm that "stereotactic radio surgery" is covered (this applies to CK, GK, FSR, Proton) is covered, but it's trying to find out what your out of pocket is.
I know others here that had microsurgery with BCBS.  Some had co-pays of 80%-20% (one even had surgery at House Clinic and still had to pay quite a bit out of pocket).
Many insurance companies look to see if you are having your treatment in-state. Sometimes that is key, unless your treating dr will verify the medical necessity of the procedure to be done out of state.
Not sure if any of this helps, but I had no issues with BCBS and my treatment.
May it go easy for you as well.
Phyl