Relm4 is actually based on gtk-rs, and you will be using gtk-rs widgets all the time.

I have to say however that despite heroic efforts by the authors, it doesn't really feel as nice as the "real" Elm architecture precisely because of the imperative focus of gtk-rs' API, the nature of Rust's memory management system, and depending on what you're doing you may feel forced far too often to drop from fantasy land to reap the benefits of the abstraction.

I'd still consider it next time I tackle a native app, but having tried Slint for writing simple UIs and having read some iced code, I think I'd only go for it if I'm really going for the Gtk native look and ecosystem.