Installing Gnome via emerge is pretty easy. I used the following command:

  # emerge gnome

Of course, this command will take a very long time to run because emerge has to emerge, compile, and install all of the dependencies and gnome itself. This is the part where you will want to go out and get a cup of coffee or take a nap.

Unfortunately, the Gentoo Handbook at the time of this writing lacked useful documentation concerning Gnome.

In order to launch Gnome instead of the X default Desktop Manager, you will need to edit your /etc/rc.conf file:

  # nano -w /etc/rc.conf
  ...
  <skipping through most of the rc.conf file...>
  ...
  XSESSION="Gnome"

Exit and save the file. When you use the startx command, Gnome will now launch