That's cool. I do appreciate the work done for GIMP/Gtk that I use. I use GNOME and other Gtk apps all of the time. And I'm only critical of GIMP because I know it could be so much better and I wish I had time to help/support GIMP and other projects like it.
All of my comments are based on user-type use and not code/API use, though I am a developer.
From my use experience I would say Gtk is usable and nice to have; since so many people can use it to make free/open software. But I would also say it is nowhere near what I would expect from a computer UI. We can do so many amazing things with software and UIs these days, but Gtk just comes across as outdated and user-unfriendly.
I hate to throw Apple into the mix, since I know it brings a lot of baggage and they aren't necessarily perfect or consistent, but their use and integration of elegant UI features is something that I enjoy and can be used as an example/reference.
I feel I'm only critical because I develop software and I believe I can imagine what a computer should be able to do. That Gtk is so useful means that did something right for developers, but I'm not convinced it is up to snuff for users--or maybe what I wish it could be.