Any new version of a program may add new bugs even as it adds new features and fixes other bugs. GCC is no exception–the GCC 11.1 release introduced Fortran syntax bugs. As Homebrew is often among the first easy sources of new GCC releases, we upgraded quickly before realizing the new bug was intolerable for us. For Intel CPU Macs, simply do:
brew install gcc@10
For Apple Silicon ARM64 Macs, there is a unique problem–GCC 10 is no longer available as binary downloads from Homebrew. This is because backports and hacks were necessary to get GCC 10 working on ARM64, and the Homebrew team wanted to encourage moving to mainstream supported GCC 11. The Homebrew maintainers provided hints for a workaround to install GCC 10 on ARM64.