Gofortran installation for mac
Yea that has always been a difficult task but I would recommend gfortran. Its part of the GCC package from gnu. It's a nice Fortran compiler and the best part is it is free. If you've searched the web you know most Fortran compilers are pricey. The link above will let you access precompiled binaries for gfortran. The binaries are packaged as simple to install Mac .dmg files.

The version for Sierra is 6.3 while the latest version for Mojave is 8.2.