I don't know. Take QT for example, I'm happy to risk blowing my leg off with c++ just because I feel really productive with Qt, it truly is a wonderfully productive library, and the external ecosystem is supportive, whatever complex visualisation I want to draw, I can often find a lib that's happy to draw into a qpixmap - c++ on its own or with boost etc. doesn't leave me feeling as productive.
For writing GTK apps the language Vala seems like a good choice. It's a C# like language whose object system uses glib objects underneath the hood, this makes object-oriented C code (as required by GTK) look like a modern object-oriented language.