GTK+ is optional, as is libglade and gtkglext...
FYI, I updated the ebuild for 0.9.6 and made these features optional. See bug 321511. If the ebuild is acceptable, this bug can probably be closed.
The problem is that they are automagic deps so you should patch the configure.ac file to fix the problem.
As long as there are no configure switches we can't do this, as it would introduce automagic dependencies. I don't see a strong need for this. If people want this please submit patches to upstream to introduce configure flags for these options.