I'm not a gimp user, Airith, so I can't speak directly to this--although some of our GIMPgurus (namely, RobA) have adapted successfully the procedures from PS, so it must be possible.
I would suggest you give it a shot & whenever you get stuck, post either in this thread or start a new one and ask for help. I can all but garuntee you'll get it!
Good luck, and sorry I'm not more help!