$ sudo apt-get update
$ sudo apt-get install libgrafx11-1-gfortran 

libgrafx11-1-gfortran is installed in your system.

Make ensure the libgrafx11-1-gfortran package were installed using the commands given below,

$ sudo dpkg-query -l | grep libgrafx11-1-gfortran *

You will get with libgrafx11-1-gfortran package name, version, architecture and description in a table

libgrafx11-1-gfortran package basic information: