I'm not sure there is as much in that distinction as you hope for 2 reasons: 1) what is the actual stability guarantee of GCC extensions? 2) what is the cash value of your distinction to the point I was making?
Re: 1, do you not believe GCC has and would modify or remove an extension which conflicts with C standard behavior? I know of instances where GCC C++ definitely deprecated an extension after it was implemented according to the standard.
Re: 2, my actual point was it's fine, even good, that that that Rust takes time. And Rust deserves that time, especially if C features made their way into the kernel via GCC C extensions. If I were a company concerned about writing a Rust kernel driver or not -- I'd be way more concerned about the vicissitudes of more wild ass kernel maintainers, Linus, and the GCC maintainers, than whether Rust is going to stabilize any necessary items on the kernel team's list.
So, IMHO, yes, there may be some difference between GCC's extension stability guarantee and Rust's unstable (duh), depending on what you want, and how you want to look at it. But at 10,000 feet, this is a mostly lawyerly, but, trivial point.