The goal of this repository is to host GNU Fortran packages for macOS. These are simple installers, that will install the GCC compilers (including gfortran) in /usr/local/gfortran.
sudo rm -rf /usr/local/gfortran /usr/local/bin/gfortran
sudo pkgutil --forget com.gnu.gfortran
If you are interested in how these installers are built, the documentation is here.