Gtk2.scm and gtk3.scm only contain the runtime libs and binaries. Their full package is available in the scm source area, named gtk2-dev.tar.xz and gtk3-dev.tar.xz. This is simply a tarball of the complete build. Simply unmount gtk2.scm and untar gtk2-dev.tar.xz in the /apps directory, it will untar as gtk2/.
Gtk2.scm and gtk3.scm have the runtime files of Xorg-7.6-lib.tcz included. Their full tarballs do not, you will need to install Xorg-7.6-lib.tcz and Xorg-7.6-dev.tcz as build dependencies along with compiletc.tcz.
The trimming of gtk2.scm and gtk3.scm was done to save bandwidth and disk space, and it should be easy to build against their full packages with the above instructions. Let me know if there are any questions.