$ sudo apt-get update
$ sudo apt-get install libgeant321-2-gfortran 

libgeant321-2-gfortran is installed in your system.

Make ensure the libgeant321-2-gfortran package were installed using the commands given below,

$ sudo dpkg-query -l | grep libgeant321-2-gfortran *

You will get with libgeant321-2-gfortran package name, version, architecture and description in a table

libgeant321-2-gfortran package basic information: