Detailed information on building gtk+ applications on OS X (native) can be found on the page GTK+/OSX/Building. We use the same build process for gedit, i.e. we use jhbuild. gedit ships with a custom moduleset which allows you to easily build the latest gedit stable release on your own. Please follow the instructions in the osx/README file shipped with gedit.
Binary images (dmg) for various OS X distributions are available on the GNOME ftp: http://ftp.gnome.org/pub/GNOME/binaries/mac/gedit/