Here is a guide to install Gnome Classic On Ubuntu 13.04, you can enable Gnome Classic by install the gnome fallback session.
Gnome Classic is the Gnome2 desktop environment used in the earlier versions of Ubuntu before Ubuntu started using Unity as the default desktop environment.
Please note that when you have MDM installed the fallback session is already installed on your system
To install Gnome Classic open a terminal window (press Ctrl+Alt+T) and copy+paste the following line:
sudo apt-get install gnome-session-fallback
When the installation is finished you can close the terminal window.
To enable Gnome Classic log out and on the logon screen, click on the Ubuntu logo and select the Gnome Fallback session. When select log back in and there you go… the good old Gnome Classic is ready to be used.