Until barely a couple of years ago, nobody wrote GUI applications in Rust anyway.
GTK's OOP semantics have been mapped very successfully to, among other languages, C++, Python, Perl and Go. It's unfortunate perhaps that the Rust bindings do not feel idiomatic for Rust programmers, but that's hardly fatal or unexpected.
I think it's just that Rust doesn't have inheritance/subclassing as a feature. As a result you have to do all sorts of mental gymnastics with impls and macros to fit Rust interface on GTK, while with other languages subclassing just "works".
I also think this is why Rust has lagged heavily on the GUI development front. No matter how you claim component or ECS-based architectures are better and faster, sometimes the only thing you really need might be a good ol' inheritance-based scenegraph... You can definitely do UI with a component-based object model (ex. Unity game engine), but the whole process seems a bit more... convoluted.
We have moved as an industry from "subclassing works great for GUIs" to "let's use it for absolutely everything else" through to "subclassing is a terrible idea, what were we thinking of?", whereas the first of those ideas is actually still sound.
GTK's OOP semantics have been mapped very successfully to, among other languages, C++, Python, Perl and Go. It's unfortunate perhaps that the Rust bindings do not feel idiomatic for Rust programmers, but that's hardly fatal or unexpected.