Since the 2.8.2 version GIMP runs on OSX natively. No X11 environment is required.
Currently there are two main installers:
A GIMP 2.8 DMG installer by Sven Claussner, stock GIMP build without any add-ons. It works on OS X 10.6 Snow Leopard and later. Just open the downloaded DMG and drag and drop GIMP into your "Applications" folder.
A build by Simone Karin Lehmann, with some extra plug-ins and changes.
A (easy?) way to compile and install GIMP and other great Free software on your Mac
is by using Macports. The installer
allows you to choose from a large directory of packages. To install gimp
using Macports, you simply do
sudo port install gimp once you have
Disclaimer: we have not investigated if it's feasible to build GIMP 2.8 from Macports. The GIMP port file mentions 2.8.10, and appears to be maintained, however.
Homebrew is similar to Macports and provides packages (aka formulas) to install, either by compiling them from source or by using pre-made binaries. While there appears to be no formula for GIMP itself, there are reports that all dependencies are available. This may help to build GIMP from source.
Fink is a package repository that offer mostly precompiled
binaries. It provides the apt-get command known to e.g. Debian and
Ubuntu users, and installing GIMP is as easy as
install gimp once you have installed the Fink
If there's no binary package, then
fink install gimp
will compile GIMP from source.
Disclaimer: we haven't been able to determine if it is possible to install or build GIMP 2.8 from Fink. GIMP 2.6.12 appears to be the most recent GIMP package that is offered there.