$ sudo apt-get update
$ sudo apt-get install gfortran-6 

gfortran-6 is installed in your system.

Make ensure the gfortran-6 package were installed using the commands given below,

$ sudo dpkg-query -l | grep gfortran-6 *

You will get with gfortran-6 package name, version, architecture and description in a table

gfortran-6 package basic information: