You must run this as sudo:
# add-apt-repository ppa:gnome3-team/gnome3 && apt-get -f update # apt-get -f install gdm gnome-shell gnome-tweak-tool
You will be asked to configure the display manager and you must choose gdm (however there is no need to uninstall lightdm, I still use it because I'm running plymouth).
Once you restart, you will see the gdm login screen, and there you can choose gnome instead of unity. This also means that you can leave that desktop environment, so you can switch to it whenever you want.
Then you will be under gnome3 desktop environment, which is quite raw, so I recommend you to install some useful extensions to improve gnome3 behavior → http://extensions.gnome.org/