Original Nautilus and early GNOME developer here. I am very out of touch with what is current, so excuse some possibly ignorant questions. How is this a Rust-based environment if it is based on GTK? I assume GTK is still essentially the C-based GTK we used with some improvements. Why is this called Rust-based? I’ll do some more research but would like to get some insight from more knowledgeable sources.
GTK is currently the best GUI toolkit for Rust application developers. It's the only toolkit that's fully functional with first class and official bindings. It's been pretty well endorsed by GNOME for most of their new applications lately.
There are some Rust GUI toolkits out there that are shaping up, such as some former Qt developers actively developing sixtyfps, but it'll be a while before we have a GUI toolkit in the Rust space that's truly ready for complex application development.