Gccrs is valuable for various reasons (bootstrapping, refining the spec, etc), but for the main purpose of "compiling to targets supported by gcc but not llvm", it's definitely not as promising as https://github.com/rust-lang/rustc_codegen_gcc. It's a rustc backend, at the same level as llvm or cranelift, it's much further along than gccrs (already available as a rustup component and can compile any rust code) and will not suffer from lagging behind rustc.

But OP has wider ambitions: the real niche targets are supported by neither gcc nor llvm. A rust-to-c transpiler that can adapt to weird C compilers should have a very wide reach.