.GIH is a GIMP brush pipe. You should be able to use it. My computer won't let me save things as .GBR if it's greyscale (which is odd), so I always save them as .GIH instead, and it always works out just fine for me. Are you sure you're putting them in the right folder and refreshing your brushes dialog box (if you haven't closed and reopened the GIMP yet)?
Yeah, I think I recall seeing that tutorial on GimpTalk one time. I shall try it out sometime, though grunge and abstract aren't really my style usually. Maybe I'll just make some from the tutorial, add some of flare (so they're original), and make them available to people on here.