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

gfortran-multilib is installed in your system.

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

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

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

gfortran-multilib package basic information: