How to switch from unity to gnome

Step 1 : To get started, press Ctrl + Alt + T on your keyboard to open Terminal. When it opens, run the commands below to enable Gnome session

sudo apt-get upgrade
sudo apt-get install gnome-session-flashback

Step 2 : Restart and on the logon screen, click the option to change your session.

Step 3 : Then choose GNOME Flashback (Metacity)

Enjoy your Desktop

Source : How to switch to GNOME Desktop Environment in Ubuntu 14.04 Trusty Tahr

The document you refer to, is many years old. In modern versions of Ubuntu, there is a small button next to your user name. When you click it, you get a list of sessions to choose from.

However, there seems to be a small bug in 14.04 that prevents that list from being shown sometimes. If you select another user and then back to your own user again, it works. I'll update this with a link to the bug report when I've filed on. I just noticed this myself so I haven't had the time yet.

EDIT: I filed it here; https://bugs.launchpad.net/unity-greeter/+bug/1309846

If you have something to add, do it there. If you're annoyed by this bug, click on "This bug affects me" in order to bring the heat up.