This build is a fully universal binary of Iain Sandoe's release of GNU Fortran 12.2-darwin-r0 for macOS 11 (Big Sur) and higher.
It installs into /opt/gfortran and can be run with /opt/gfortran/bin/gfortran.
Fully universal means that it runs on both arm64 and x86_64 architectures and it can produce code for both arm64 and x86_64 (technically, it consists of two native compilers and two cross-compilers - see build/README for details).
The /opt/gfortran/SDK symlink should point to the desired SDKROOT (defaults to the command line tools SDK). This can be updated with /opt/gfortran/bin/gfortran-update-sdk or manually. If the symlink is broken, the driver will issue a warning and use xcrun to try to find an SDK and set SDKROOT accordingly.
Note: Only the driver-driver gfortran (which dispatches to the corresponding *-apple-darwin20.0-gfortran driver) has a name without the architecture and build. It does not support in-flight lipo (i.e. multiple -arch switches at once), so either use the experimental driver shell script or compile for each architecture separately. Other tools don't have driver-drivers, but can be used via the full <arch>-apple-darwin20.0-<tool> name if desired.
(The binaries in the releases 12.2-darwin-r0 and 12.2-darwin-r0.1 are identical with the exception of the gfortran driver-driver.)