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...
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.
AzPainter is one more app which focuses quite specifically on digital painting and art.[0]
Which ever you prefer, open source developers have created some great graphics software (and these two are far from the only examples).