All of the GIMP brushes that I have are ones that I've raided off of deviantART. I would check there for the GIMP brushes. But many credits go to the PS to GIMP brush converter dude for some awesome GIMP brushes that he took from PS brushes.
Yeah, TBQH I don't remember who made most of the ones I've got, so it really wouldn't be right to release them without credit. BUT I can give you a little help- Clicky