This simple and brief tutorial will show you how to install Gnome Shell on Ubuntu 12.04 Precise.
GNOME Shell provides core interface functions like switching windows, launching applications or see your notifications. It takes advantage of the capabilities of modern graphics hardware and introduces innovative user interface concepts to provide a delightful and easy to use experience. GNOME Shell is the defining technology of the GNOME 3 user experience.
Open a terminal by pressing CTL+ALT+t,then run below command to install GNOME SHELL
sudo apt-get install gnome-shell
Then log out from Unity ,click the GEAR button to log in using GNOME SHELL
You are done,enjoy.