This is GNU Fortran (GCC) 12.0.1 20220312 (experimental) build of the arm64 from https://github.com/iains/gcc-darwin-arm64 (based on 935876a) as used for the R 4.2.0 Big Sur arm64 release.
This binary installs into /opt/R/arm64/gfortran with an additional symlink in /opt/R/arm64/bin/gfortran. To unpack use
tar fxz gfortran-12.0.1-20220312-is-darwin20-arm64.tar.xz -C /
(prefix with sudo if you need admin permissions to write to /opt/R/arm64). To use set
export PATH=$PATH:/opt/R/arm64/gfortran/bin
In addition, /opt/R/arm64/gfortran/SDK has to point to your macOS SDK. The binary above points to MacOSX11.sdk of Xcode Command Line Tools, but you may need to change it to match your installation. We include a script gfortran-update-sdk in the above tar ball that you can simply run (prefix with sudo if you require admin permissions).
This build has been configured with:
../gcc-darwin-arm64/configure --prefix=/opt/R/arm64/gfortran --enable-languages=c,fortran \
--with-gmp=/opt/R/arm64 --with-mpc=/opt/R/arm64 --with-mpfr=/opt/R/arm64 \
--with-sysroot=/opt/R/arm64/gfortran/SDK
Where dependent libraries have been built using the recipes system - see also https://mac.r-project.org/bin/.