Btw there is a new fork of GIMP, called "Glimpse" [1], that tries to improve the UI. Their first release was just a re-branded GIMP, but maybe they'll get something done someday.
Select circle brush, set outer size, paint circle, set inner size, erase circle.
Not that the UI isn't terrible.
Edit:
Simpler than this or the linked instructions is to use the Ellipse selection tool with shift to select a circle, then do Edit > Stroke Selection.
Still pretty wacky.
(More and more graphics programs are adopting it.)
Once you're used to it, you can create the ellipse selection and type "/stroke"...
I guess that unless the Gimp developers change their mind and allow for vector drawing inside of Gimp, it's as comfortable as it can get...
AzPainter is one more app which focuses quite specifically on digital painting and art.[0]