This is an automation-friendly re-packaging of the 8.2 release (exactly the same content) for use on GitHub workers. It is a lot smaller than the dmg and faster to install. It works on macOS High Sierra and higher (even though it was built for Mojave). It contains just the /usr/local/gfortran tree.
Suggested installation:
curl -sSL https://github.com/R-macos/gfortran-for-macOS/releases/download/8.2/gfortran-8.2-Mojave.tar.xz \
| sudo tar fxz - -C /