Most server admins do not run a GUI.
But if you want one for some reason, you can install any GUI you like: Unity, Gnome, Xfce, KDE, etc.
For example to install and launch the lightweight "fluxbox" windows manager:
sudo apt-get update
sudo apt-get install fluxbox
Alternately, you might consider a technology such as Webmin, which allows you to administer your server through a control panel in your browser:
Last but not least, here is a link to the required reading
"Ubuntu Server Guide" which will teach you to administer your server the recommended way (no GUI necessary):