There's two different methods by which Rust support can be added to GCC: adding a Rust frontend to GCC and adding a GCC backend to the Rust compiler (rustc_codegen_gcc). The latter approach would not be (as?) susceptible to implementation divergence as an independent frontend.

I hope for the former since then the functionality might become available as attributes in C too.

yep, if git is content with rustc_codegen_gcc, then it's very doable they can require rust in the next few years