I used to like C language, because it is a basic of programming, and it is portable, and it is low-level. When writing program with C language, it is just like showing off your advanced programming skill, how you manage the memory, how you manage the pointers and creating the linked list. However, in terms of efficiency, C++ is much more powerful, because of object-oriented and the syntax.
Because I like C language, so I chose GTK+ over Qt for long time ago. Not only that, I am also fond of GTK+ desktop environments like GNOME, Xfce4, LXDE, Cinnamon, but not Mate. I feel that KDE is heavy weight.
One of my personal projects, Med, which was written with GTK+, had some issues with multi-threading. I believe that my engine part does not have any problem. As a result, the suspicious part is the GTK+. The program crash without symptoms, and difficult to reproduce. Therefore, I decided to re-write the UI with Qt. If, Qt also produces the same problem, meaning my algorithm is problematic.
In my opinion, GTK+ is straight forward, because of procedural paradigm. Therefore, it is easy to learn and implement. The UI can be design with Glade. But I feel that it is still lack of something, such as button click callback function. Besides that, though there is gtkmm (C++ interface of GTK+), the library like WebKitGTK+ does not have the documentation for gtkmm (I believe still can work). In summary, GTK+ is slower as writing the code in C language.
On the other hand, Qt is more complicated. To use it, better to use qmake project file to generate the Makefile, or use the CMake, so that we can include the necessary headers and link with necessary libraries. And we also need to use some macro. This makes me feel that the coding is very Qt-oriented, not just a C++ language. However, in terms of design, I feel that Qt Designer is much easier. But GTK+ and Qt layout uses different concept.
Though GTK+ and Qt are both object oriented, GTK+ is in the library level yet Qt is in the language level. It is easier to write inherited classes in Qt, and easily make changes at the language level.
As a conclusion, Qt is better than GTK+ for development as C++ is better than C.