$ sudo apt-get update
$ sudo apt-get install libmathlib2-gfortran 

libmathlib2-gfortran is installed in your system.

Make ensure the libmathlib2-gfortran package were installed using the commands given below,

$ sudo dpkg-query -l | grep libmathlib2-gfortran *

You will get with libmathlib2-gfortran package name, version, architecture and description in a table

libmathlib2-gfortran package basic information: