Ubuntu 11.10 no longer includes Gnome 2. Gnome 3 is included, but it is rather different from Gnome 2. Although there are ways to install Gnome 2, I doubt you can achieve good system stability with them, given that Unity runs atop Gnome 3, and Gnome 2 is no longer maintained. You can install Xfce, however. Enter this command in the terminal and then enter your password when prompted.
sudo apt-get install xubuntu-desktop
Xfce is different from Gnome 2 in some ways, but it is closer to Gnome 2 than anything that you can easily install. Click the gear next to your username when logging in, and select the Xfce option.