All the news about GTK were about big projects dumping it in favor of Qt or other, did anything changed significantly? I only seen it used cross platform for extremely basic GUIs for Python apps. It also looked like shit==GNOME on Mac and Windows.
Meanwhile QT continues to work great and they've largely solved the licensing issues that were a problem in the past. It's also a comparatively stable project and supports targets like android, iOS, rendering directly on to buffers (you can embed it into games). If you're starting a new project I think QT should be your default unless you have a good reason to use GTK.
I'm not qualified to assess the code quality of either project, but I have been impressed by QT for the few things I've needed to build a gui for. Things just work but if you need really specific functionality (like rendering directly to a framebuffer with no display server on embedded linux) it's not too hard to make it work. I'm honestly a bit mystified as to why anyone would choose GTK for a new project, unless they were specifically targeting the Gnome desktop and only the Gnome desktop.
Sort of?
Mostly just design patterns that are explicit to GNOME. And that is primarily a response to people being (rightfully or not) unhappy that too much GNOME was ending up in GTK itself.