[QUOTE=Octal]Is GUI programming without GTK possible? If so, how?[QUOTE]
#include <X11/Intrinsics.h>
and use the Widgets.
GUI on Unix is normally done with X11, this is a layered architecture with Xlib at the bottem, which gives you a raw C api to the protocol, then Intrinisics which give you Widgets which are basically object orientated constructs supporting windowing on X11. Start with 'man XtCreateWidget'.
If you want a comparison using Xlib and Intrinsics is comparable to using the Win32 API without MFC.