In well-tested libraries like GTK, SQLite, Curl, and such, they are often quite robust just based on having been heavily developed and tested by many people over a very long time, and there are still ways that they can be misused and abused that are usually well-documented and warned against. A well-developed Rust wrapper actually makes it impossible to misuse one of these libraries from Rust, and therefore better enables a much smaller team of developers to write secure, robust applications. Rust can guarantee these documented restrictions at the type level and even make impossible many error conditions.
So even though the UI is GTK, Rust still enables developers to write more robust applications with less fear. Personally, I find that GTK with Rust is a very pleasant experience. It's less about guaranteeing that the lower libraries have no bugs and more about preventing people from interacting with the libraries in dangerous or buggy ways.
While the lower layers written in C do impact the overall safety, the bindings are made to be as safe as possible.
For example: every glib parameter that may take NULL in Rust becomes an Option<T>.
GObject's methods are defined on traits and checked by the Rust type system. There are also some macros to provide an easy and safe interface to the GLib type system.
All of this directly applies to gtk-rs.
Overall, the bindings are well documented and with many examples. There's even a book. Also, there's a great community around them.
Bindings website: https://gtk-rs.org/
And it has to be said that memory safety isn't Rust's only compelling feature. It also has a pretty great build system, a decent library ecosystem, a very strong type system which gives you an "if it compiles it works" experience surprisingly often, probably the best multithreading system, etc. etc.