$ sudo apt-get update
$ sudo apt-get install gfortran-mingw-w64 

gfortran-mingw-w64 is installed in your system.

Make ensure the gfortran-mingw-w64 package were installed using the commands given below,

$ sudo dpkg-query -l | grep gfortran-mingw-w64 *

You will get with gfortran-mingw-w64 package name, version, architecture and description in a table

gfortran-mingw-w64 package basic information: