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...