WPF is really, really declarative, extensible and composable in a way I haven’t seen in any other desktop UI framework. And certainly not in Gtk.

I’m not going to say it can’t be done, but I will venture a guess that it’s going to be difficult and probably a lot of effort.

More effort than simply forking and porting to be X11/Wayland native? Not sure, but I wouldn’t dare to exclude the possibility.

WPF is built on DirectX, you would probably want to port it to something like Skia.

It's not exactly WPF, but there are folks that have worked on supporting Xamarin.Forms / XAML Standard on Gtk+: https://github.com/jsuarezruiz/forms-gtk-progress