Here is a starting point to install GTK on OSX: http://live.gnome.org/GTK%2B/OSX/And It can running separately from X in OSX?Not yet.
Actually you can run it separately from X server, like an ordinary application, when using Quartz backend. What you need is is to check GTK-OSX project, see more info here: http://www.gtk.org/download/macos.php
Basically, you need to build GTK with jhbuild scripts and bundle the GTK libraries with your program.
Regards,Miroslav