First you nee to make sure you have enabled Universe and multiverse repositories in /etc/apt/sources.list file once you have enable you need to use the following command to install GUI:
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install ubuntu-desktop gnome
The above command will install GNOME desktop on the Ubuntu server
If you want to install a graphical desktop manager without some of the desktop addons like Evolution or OpenOffice for instance, but continue to use the server flavor kernel use the following command
sudo apt-get install --no-install-recommends ubuntu-desktop
If you want to install light weight desktop just install xfce one using the following command:
sudo apt-get install xubuntu-desktop
If you want to install KDE desktop use the following command
sudo apt-get install kubuntu-desktop