This tutorial will explain how to install classic gnome desktop in ubuntu 13.04.By default ubuntu 13.04 loads in to unity desktop.If still want to use old classic gnome desktop you have to install gnome session fallback package.
Install classic gnome desktop in ubuntu 13.04
Open the terminal and run one of the following command
sudo apt-get install gnome-session-fallback
Once you install the classic gnome desktop you need to logout from the existing session.Once you are at logon screen click the option to change your session then choose Gnome Fallback to logon
Once you logged in you should see similar to the following classic gnome desktop