This is more than a bit BS, based on someone's personal preferences. Glib, on which gtk is based, is indeed fairly "object oriented". However, "object orientation" in this sense does not require class paraphernalia, nor do you need to study or reflect much on the paradigm -- it is just a matter of logic and common sense, the API provides the structure (you do not have to impose more structure on it). I have done GUI OO programming in perl, IMO it is no big advantage*. It would be just as easy to say that using C++ here will slow you down because you now need to learn a lot of bloated and arcane syntax. Like, truth be told, you will do all this WAY FASTER in a language like perl or python than you will do it in C++ or C, altho that is only in small part due to the "object oriented paradigm".
Also, I would not try and learn C++ and GUI programming at the same time. If you want to learn C++, start that. If you want to learn GUI stuff, just start now with the WinAPI.
Like I said before, you are not closing the door on anything (that's Mario's job ).
*I'm sure this perspective comes from people who have not spent any serious time doing anything but OO, so they are flabbergasted at the idea that doing without has it's own advantages and techniques.
In short, not likely (unless you are already very fluent with C++, and never do much pure C programming). Also -- I have never tried this, but I know people who have to good effect -- you can use a system called "glade" which allows for XML scripting (XML, like HTML, is very simple) with gtk+ design. That will save you way more time than the use of C++ wrappers. However, I really think you are best off using WinAPI for now, if that is the platform you use most.