sudo apt-get install gnome-shell
That is all you have to do to install Gnome Shell and Classic (Fallback mode) Gnome. This will install Gnome 3.4 so it is up to date as of now. Once it is installed just log out. On the login screen click on the Ubuntu icon by your name and select what you want to log into. Then enter your password and you will be logged in to the Gnome Desktop.