This library, when ready, will provide a GObject interface to the Linux joystick API. This should make the use of a joystick for a GTK application all that much easier.
I actually already wrote such a library back when I was first learning about GObject, now about 10 years ago. I accidentally removed the code at the time, after having spent about a day or so writing it.
The library also contains a small GTK-based joystick test application.