I don't think it covers GUI callbacks like on Gtk-rs, accessing internal widget data.

I think that's likely. async/await doesn't appear to map directly onto traditional OO GUI framework callbacks well, just like rust doesn't tend to map well to traditional OO. I believe that async/await will open up new architectural patterns that should be as ergonomic as traditional OO callbacks for GUIs. But these won't map well to existing code or existing frameworks.

There have been attempts, like relm, to map futures onto GTK. That means it would work with async/await too, as they’re fundamentally sugar for futures.

Even those are basically Relm components that happen to render via GTK. You can't really import a plain `GObject` and implement a traditional `fn on_click()` without having to worry about internal mutability and the associated borrow checking complexities.