Minimal quick start guide in order to get a working version of gimp on OSX. For this to work properly, remove all traces of any previous Macports, Homebrew, or jhbuild installation.

The dollar sign precedes any terminal commands.

Backup "~/.profile" for a fresh start.
$ if [ -f $HOME/.profile ]; \
  then \
  mv $HOME/.profile $HOME/.profile-bak \
  fi

Most OSX installations come with git preinstalled. If you do not have git installed, you can download a prebuilt binary from here:
http://git-scm.com/download/

Download and install jbuild
$ cd $HOME
$ curl -O http://git.gnome.org/browse/gtk-osx/plain/gtk-osx-build-setup.sh
$ chmod +x gtk-osx-build-setup.sh
$ sh gtk-osx-build-setup.sh

Add jhbuild install dir to your $PATH and ~/.profile
$ export PATH=$HOME/.local/bin:$PATH
$ echo 'export PATH=$HOME/.local/bin:$PATH' > $HOME/.profile

Download a custom jhbuild files for gimp
$ curl -o $HOME/.jhbuildrc-gimp http://git.gnome.org/browse/gimp/plain/build/osx/jhbuildrc-gimp?h=gimp-2-8 &&
  curl -o $HOME/gimp/directory/gimp.modules http://git.gnome.org/browse/gimp/plain/build/osx/gimp.modules?h=gimp-2-8

Now you can download and install gimp, where 'gimp' is the directory prefix, and '10.6' is the minimum version you are building gimp for. As it stands, you should be able to leave the values as they are.

$ JHB=gimp GIMP_SDK=10.6 jhbuild bootstrap

This process requires some manual intervention. When the build fails on xml-parser, open a new shell and complete the install
$ JHB=gimp GIMP_SDK=10.6 jhbuild shell
$ cd ~/gimp/source/XML-Parser-[parser version here]
$ make -j3
$ make -j3 install

NOTE: When the perl module pops up, it may tell you that it can neither make nor install. That's fine, just enter "2" and then press the enter key, and then press "2" again and hit the enter key again.

Do the same for simple-xml
$ cd ~/gimp/source/XML-Simple-[simple version here]
$ make -j3
$ make -j3 install

Now we can build gimp
$ JHB=gimp GIMP_SDK=10.6 jhbuild build gimp-2.8-python

If you would like the gtk murrine theme, you can install it as well
# JHB=gimp GIMP_SDK=10.6 jhbuild build murrine-engine

In order to run gimp with dbus support (required for gvfs and gio)
$ dbus-launch gimp-2.8

This will generate a dbus session. If you continuously run gimp using this method, you will end up with many dbus sessions being run. Kill them after all gimp instances have been killed. 

In order to create a .app you'll need to download and install gtk-mac-bundler
$ cd $HOME/source/
$ git clone https://github.com/jralls/gtk-mac-bundler.git
$ cd gtk-mac-bundler/
$ make install

To create a bundle, we need some more files
$ curl -o $HOME/gimp/directory/gimp-2.8-python.bundle http://git.gnome.org/browse/gimp/plain/build/osx/gimp-2.8-python.bundle?h=gimp-2-8 &&
  curl -o $HOME/gimp/directory/launcher-2.8-python.sh http://git.gnome.org/browse/gimp/plain/build/osx/launcher-2.8-python.sh?h=gimp-2-8 &&
  curl -o $HOME/gimp/directory/info-2.8-python.plist http://git.gnome.org/browse/gimp/plain/build/osx/info-2.8-python.plist?h=gimp-2-8 &&
  chmod +x launcher-2.8-python.sh &&
  curl -o $HOME/gimp/directory/gimp.icns http://git.gnome.org/browse/gimp/plain/build/osx/gimp.icns?h=gimp-2-8 &&
  curl -o $HOME/gimp/directory/xcf.icns http://git.gnome.org/browse/gimp/plain/build/osx/xcf.icns?h=gimp-2-8 &&
  curl -o $HOME/gimp/directory/custom/fonts.conf http://git.gnome.org/browse/gimp/plain/build/osx/custom/fonts.conf?h=gimp-2-8 &&
  curl -o $HOME/gimp/directory/custom/gimp.desktop http://git.gnome.org/browse/gimp/plain/build/osx/custom/gimp.desktop?h=gimp-2-8 &&
  curl -o $HOME/gimp/directory/custom/gimprc http://git.gnome.org/browse/gimp/plain/build/osx/custom/gimprc?h=gimp-2-8 &&
  curl -o $HOME/gimp/directory/custom/gtkrc http://git.gnome.org/browse/gimp/plain/build/osx/custom/gtkrc?h=gimp-2-8 &&
  curl -o $HOME/gimp/directory/custom/mimeinfo.cache http://git.gnome.org/browse/gimp/plain/build/osx/custom/mimeinfo.cache?h=gimp-2-8 &&
  curl -o $HOME/gimp/directory/custom/open.desktop http://git.gnome.org/browse/gimp/plain/build/osx/custom/open.desktop?h=gimp-2-8 &&
  curl -o $HOME/gimp/directory/custom/sesstion.conf http://git.gnome.org/browse/gimp/plain/build/osx/custom/session.conf?h=gimp-2-8

We also need to fix a few outstanding issues
$ cp ~/gimp/10.6/source/gettext-0.18.1.1/gettext-tools/intl/charset.alias ~/gimp/10.6/inst/lib
$ chmod +w $PREFIX/lib/libpython2.7.dylib

Now we can run the bundler
$ cd $HOME/gimp/directory
$ gtk-mac-bundler gimp-2.8-python.bundle

Please note that this is not a flawless process, and there are a few issues that still need to be fixed. Please join #gimp on irc.gimp.org if you have any questions.