Hello @lynd, Welcome to Connect. Headaches can be a symptom of prednisone withdrawal but I'm not sure of sensitive scalp. My doctor was always asking me if I had any tenderness in the scalp or temple areas due to the concern of GCA. I don't think blood tests alone can diagnose GCA. Here's more information on the topic:

"How is GCA diagnosed?
The best way to confirm a diagnosis of giant cell arteritis is by taking a small sample (biopsy) of the temporal artery. This artery is situated close to the skin just in front of your ears and continues up to your scalp."
-- Giant cell arteritis - Diagnosis and treatment:
https://www.mayoclinic.org/diseases-conditions/giant-cell-arteritis/diagnosis-treatment/drc-20372764.
@tsc and @zaa may also have some thoughts on headaches and sensitive scalp while tapering to a lower dose of prednisone. Have you tried bumping the dose back to 6 or splitting a 1 mg tablet and going up to 5.5 to see if the symptoms go away?