I, as well as many others, would adamantly disagree. For many, many, developers then principle reason to use Gtk+ is that it is all C. Besides the preference of language styles, C instead of C++ for libraries has countless advantages from a software engineering standpoint from compiler binary compatibility, to the techniques used build wrapper interfaces for higher level languages. Many get this point, yet still want a GUI library in C++, not thinking about times when you want to create a library that itself uses a gui toolkit.
I also would not recommend libglade or gtkbuilder.
Qt is not really C++ anyway, it's c++ and a macro language. (which in and of itself illustrates some of the limitation of C++ in the first place)