Though I use Qt, the Mac support on GTK is terrible at best. Windows support is even worse (or non-existent) which makes me question their "cross platform gui" title.
If I was to suggest a method, I would suggest using each platforms GUI language and make a backend in something cross platform.
Running on multiple Linux DE's/OS's is not cross platform as far as I am concerned, even though technically it is lol.
"Cross platform" means supporting all of the widely-used platforms at a given point in time, and supporting them well. Today, that includes at least Windows, OS X, and Linux. Some would even extend that to include the BSDs, Solaris, AIX and HP-UX.
Like others have pointed out, GTK+'s support for OS X and Windows has been very, very lacking. It's nowhere near as seamless as that offered by Qt, Swing, or SWT.
Qt has just about as many bindings as GTK does.
http://www.gtk.org/language-bindings.php - 3.10 - 12 http://en.wikipedia.org/wiki/List_of_language_bindings_for_Q... - 7
And of that 7, 3 are the only "language integrations". The rest are parts (Go-QML)/part of Qt (QtQuick, C++).
So 12 to 3 for GTK. And the GTK language binding is not complete, for example Rust has a GTK binding (though not done).