I don't have gimp installed on my mac yet, however, I've known about and used gimp on my old windows pc ever since my son introduced me to it several years ago. (maybe around 2006 or so...)
I remember being pleasantly surprised with gimp back then. (I just got this mac and am wading in.)
I've read some posts on here today that mention getting the mac download from various servers like lisanet, and I will probably download it after a little further reading.
I'm still in the middle of reading the forum's suggested links so that I can get back up to speed with the program.