$ sudo apt-get update
$ sudo apt-get install gfortran-5-multilib 

gfortran-5-multilib is installed in your system.

Make ensure the gfortran-5-multilib package were installed using the commands given below,

$ sudo dpkg-query -l | grep gfortran-5-multilib *

You will get with gfortran-5-multilib package name, version, architecture and description in a table

gfortran-5-multilib package basic information: