@Iains has some work in progress on https://github.com/iains/gcc-darwin-arm64 to port the GCC backend to Apple Silicon.
Mind that Apple Silicon support is going to require GCC 11 even in the best case. The first stable release of GCC 11 may come out in mid-2021 or later. If you absolutely require a stable GCC, or any formula that depends on it, you may want to hold off your Apple Silicon Mac purchase decisions until it’s clear if or when GCC will support it.
For limited testing on Apple Silicon, Homebrew may consider shipping an unstable GCC 11 but that’s yet to be decided.