The advantage that Rust's GTK bindings have over Rust's Qt bindings is great enough to counteract any advantage that Qt has over GTK. That's because GTK has an (XML?) schema that allows bindings and documentation to be autogenerated to a large extent. Binding to Qt from Rust is in comparison very difficult (not to mention the fact that Qt's idiosyncratic programming model naturally conflicts with Rust's militant ownership concepts).

Edit: To be clear, I'm not comparing the relative merits of different GUI options, I'm saying that GTK is the only option that you really have in Rust today.