$ sudo apt-get update
$ sudo apt-get install phonon-backend-gstreamer 

phonon-backend-gstreamer is installed in your system.

Make ensure the phonon-backend-gstreamer package were installed using the commands given below,

$ sudo dpkg-query -l | grep phonon-backend-gstreamer *

You will get with phonon-backend-gstreamer package name, version, architecture and description in a table

phonon-backend-gstreamer package basic information: