So if I get it right on this one using C++ would allow me to design and code a GUI faster then I would be able to do it in GTK+?

Next to that what is mostly used by company's these these to code software for their clients that need a GUI, C, C++ or something different?

I am currently learning C. But I suppose that catching up with C++ later on after the book MK27 referred me to would be easy enough with a basic understanding of the C language?