$ sudo apt-get update
$ sudo apt-get install gksu-polkit 

gksu-polkit is installed in your system.

Make ensure the gksu-polkit package were installed using the commands given below,

$ sudo dpkg-query -l | grep gksu-polkit *

You will get with gksu-polkit package name, version, architecture and description in a table

gksu-polkit package basic information: