My guess is that Gentoo sets it up so that only root can shutdown the machine. It is a security feature. So try this...it is from member sbing in a Slackware FAQ (I edited it to reflect non-slackware-centric principals).
This command requires you to have the "sudo" command. I assume you have it installed:
visudo /etc/sudoers (really, any text editor should work)
Place the following in the file:
yourusername yourhostname = NOPASSWD: /sbin/reboot, /sbin/halt -h now, /sbin/halt
You can now shutdown via sudo without a password.
To enable shutdown/reboot from within gnome (ala the way DL GNOME sets it up):
mkdir /var/run/console
touch /var/run/console/yourusername
chmod +s /sbin/reboot
chmod +s /sbin/halt
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