I think the terms "easy to use GUI" and C language are incompatible. Made worse by the idea that you probably want the GUI to be OS-platform independent. Of course, its all software, so anything is possible.
Back in the 1980s, I wrote C code for Windows 386. It was possible, but it sure was not easy to use.
I rarely see "C" and "GUI" in the same set of requirements. While there is tons of C being written in the embedded space, that world does not do WIMP GUI work.
I agree that "easy to use" and C (or C++) don't go hand in hand, at least not in the world of free software. (Some of the commercial GUI libraries may be easier to get started with.)
Personally, I don't think that X11/Xt is hard. It's obsolete, the code is certainly verbose, and it's not fun to program - but X is widely available and free. It all depends a bit on which platform the code is supposed to run on, and how you intend to distribute the application (if at all).