Free FORTRAN compiler - or one that does not have to be updated all the time
re free Fortran compilers

gfortran is part of gcc (gcc stands for GNU Compiler Collection not just the name of GNU's c++ compiler)

Xcode now does not install gcc (even via command line tools) gcc is a link or wrapper to clang

As to installation my first attempt would be via a package manager like Macports or Homebrew or fink.

For macports (others will be similar) install will be


Install macports as per the guide
install gcc with the fortran option sudo port install gcc5
install a select program to allow selection of different gcc versions (in this case there will only be one) sudo port install gcc_select
select the version sudo port select --set gcc mp-gcc5


gfortran is now in /opt/local/bin

Macports should download readymade binaries for this so no wait for a long compile of gcc

If you upgrade the OS Macports will need a complete reinstallation (OK it might work but support and test is only done on a complete reinstallation, previous OS upgrades have definitely needed one the last 1-2 might have worked)

Homebrew instructions are included in this article which also includes other free Fortran compilers and other useful Fortran info.