There are many ways to run gimp under Mac OS X. Why not using the easiest one, looking for gimp on
www.macupdate.com ? I have version 2.2 running without a problem (although I have to admit, I'm not using it much).
There also is a gimp version in prefix portage (version 9999). Maybe you should do an "emerge --sync".
Obviously (by looking at usr/portage/x11-base/) there is currently no general X11 support in prefix portage, but gimp itself uses gtk rather than x11, and qt seem also to compile fine (emerge currently runs in background here) - and, given all the mac specialties (interfacing with mac os x aqua) I don't know, if it is worth the effort.
I do cross-development on the mac, running Linux(Solaris, etc.) in virtual machines, working via ssh and starting X-apps when needed. There is only very rarely any problem with X. It is stable enough, to do everyday development work.
If you do much graphics under X11, YMMV.