he portability of the Windows and OSX UI frameworks could properly be called "utter rubbish", because they're not intended to be portable at all.
What exactly do either of those have to do with a discussion of Qt vs GTK+?
In contrast, GTK+ apps can and do run on both Windows and OSX, and many applications work quite well on both platforms.
I've use GTK+ apps on both Windows and OS X and they do not work "quite well". Many OS X GTK+ apps still require pulling in X11 which adds extra hassle and more dependencies whereas Qt does not have that issue (even if Qt apps still don't look completely native on OS X).