Linux GUI development [closed]
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.
Gnome apps work on KDE desktops and vice versa; you won't be locking anyone out. As far as toolkits go, it's fairly subjective. All of the toolkits are fairly cross-platform. If you're not open source, then GTK+ would be the cheaper option, as Qt is only free for open source use, whereas GTK+ is LGPL.