I have not tried GTK because I cannot fathom why would anyone use a UI toolkit written in C. But even language trash talk aside, it's probably easier for a C++ project to use a C++ GUI library instead of a C one albeit with C++ bindings. Also I've read somewhere that it the C++ GTK bindings are kinda sucky.

These are just my assumptions, though. Someone who has tried both GTK and Qt might give a better answer.