https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121632
--- Comment #3 from Sam James <sjames at gcc dot gnu.org> --- It looks like they use specific versions of GCC and then apply patches on top: https://github.com/Homebrew/homebrew-core/blob/main/Formula/g/gcc.rb#L12. For --HEAD (I'm not an expert on homebrew), it looks like they have no handling to use Iain's fork where appropriate. You need to find some way to override the git repo it uses.
