For some reason, the default editor in Gentoo is Nano instead of vi or emacs. To change the default you need to edit your rc.conf file. You should see something like the following in the file:

 # Set EDITOR to your preferred editor.
 EDITOR="/bin/nano"
 #EDITOR="/usr/bin/vim"
 #EDITOR="/usr/bin/emacs"

To change your editor, add a pound sign (#) in front of the EDITOR="/bin/nano" line and remove the pound sign at the beginning of the line that contains the editor you want to use. If you want to use an editor other than vi or emacs, you can add new line with the path to the name of your editor.