GNOME is a feature-rich desktop provided by the GNOME Project. It is one of the most widely-used desktops for Linux. It has been an important part of Gentoo for many years.
See the GNOME Guide for more a more detailed approach on installing, configuring, and using GNOME.
GUI user's guide
GNOME 3's GUI is a total upheaval for users of GNOME 2. Getting familiar with it, at least for its basics, is a matter of minutes provided the user is directed to the right entry point. A rather good entry point to quickly get familiar with this new GUI is the GNOME Help page.