Linux GUI development

I recommend wxWidgets or Qt. They are both mature, well-structured and cross-platform, with decent documentation and sample source code.

Your best bet may be to port it to a cross-platform widget library such as wxWidgets, which would give you portability to any platform wxWidgets supports.

It's also important to make the distinction between Gnome libraries and GTK, and likewise KDE libraries and Qt. If you write the code to use GTK or Qt, it should work fine for users of any desktop environment, including less popular ones like XFCE. If you use other Gnome or KDE-specific libraries to do non-widget-related tasks, your app would be less portable between desktop environments.