libui only wraps around GTK+ on Unix systems. On Windows, it uses the native Windows API directly.

GLib is a utility library; it provides things like container types, filesystem access, networking utilities, threads, object-oriented programming with events and properties, and other similar functions in a platform-independent manner. GTK+ uses GLib. You can use GLib on its own as well. It won't solve the problem of GTK+ being clunky on Windows, though.