To build GIMP.app from source:

1) install macports from macports.org

2) add +quartz +atsui +no_x11 -x11 to /opt/local/etc/macports/variants.conf

3) run:
   sudo port -d sync
   sudo port install gimp2 +without_gnome -dbus
   sudo port install gimp-user-manual macfile-gimp macclipboard-gimp icns-gimp

4) download GIMPskel.zip from gimp-app.sf.net and extract

5) load ScriptExecCocoa/ScriptExec.xcodeproj with Xcode and click build

6) run gimpguts.sh