I can’t speak to Rust+GTK, but GTK is old enough and stable enough there’s no reason it shouldn’t be stable. As for advice on learning GTK, the docs are decent, stackoverflow often has solutions to the non-obvious bits. I don’t ever do anything fancy, so I don’t have a lot of additional advice.
Another option to consider, if you want to develop cross-platform stuff, is WX. It’s a wrapper over GTK (on linux, or whatever the windows and mac equivalents are), I used it about 10 years ago when I was doing UI work on a regular basis. It handles most of the tedious bits for you, while still letting you access the underlying properties when needed (which should let it rescale for phone-displays without too much effort).
I’m in a late batch I’m sure (ordered Jan-2019), so it’ll be a while before I can do anything myself, but I figure I’ll do some playing with it when I can. I may get impatient and grab the qemu image when the first physical devices ship.