Danger Will Robinson!
By default Ubuntu does not use the "Root" system. It uses sudo instead. There are different explanations for what it means, but commonly it stands for
superuser
do.
When you use sudo it
temporarily gives you "Root" rights for 15 minutes. The prompt will not change. It will remain the same.
Try it now e.g. with the command: sudo apt-get update
Have a look starting here:
http://ubuntuguide.org/wiki/Ubuntu:Trusty#Introduction
There are times when gksudo is recommended instead of sudo, generally when you are opening a graphical application. An alternative explanation to the ubuntuguide is here:
http://www.psychocats.net/ubuntu/graphicalsudo
Yes, as atanere says, you can use sudo su, but it is pushing Ubuntu to a place it does not normally go. If you can avoid it...don't. Mostly there should be no need to change from sudo.