$ sudo apt-get update
$ sudo apt-get install gnustep-back0.20-art 

gnustep-back0.20-art is installed in your system.

Make ensure the gnustep-back0.20-art package were installed using the commands given below,

$ sudo dpkg-query -l | grep gnustep-back0.20-art *

You will get with gnustep-back0.20-art package name, version, architecture and description in a table

gnustep-back0.20-art package basic information: