I think it's important to have multiple implementations of Rust, and to support as many platforms as possible, and as @cuviper suggests, reusing the reference front-end and plugging in a gcc backend may be the most productive short-term path. There is one other project intending to create a new Rust backend, and it's important that rustc support the use case.
The major blocker is refactoring rustc to support non-LLVM backends, after that it's just an enormous amount of work.
It's hard to predict the things that Mozilla will support financially, as there are many sources of funding within Mozilla and they all have their own opinions. I personally think it's a worthy effort, though the importance of a gcc backend at this time is relatively low compared to other projects that are currently underfunded. For the purposes of platform support it may be easier to pay an LLVM developer to write LLVM backends for specific CPU architectures than to write a new rustc backend.