At this point the MacOSX port of GTK+ is still in a beta stage and is not ready to be used for production work. There is a framework build installer available on the page you referenced. However it is Intel only and judging by the posts on the forum at the developer page,
http://developer.imendio.com/forum/gtk-macosx , there will not be a Universal build or a separate PowerPC build anytime soon. That aside, GtkGLext has not been ported to MacOSX yet
http://developer.imendio.com/node/226 so it would not be possible to build GnuBG with the 3D board. On the whole I believe that trying to build GnuBG with the MacOSX port of GTK+ at this time is opening up a big can of worms. It is not really necessary since as others have pointed out GnuBG on MacOSX-X11 is very easy to set up and run and is very stable. The native port should be left for the hopefully near future when all the kinks are worked out. For now really all that is required is a bit of hand-holding for the Mac folks who need help setting things up.