How route (masquerade) internet on Sabayon 7 (Gentoo base distro)?
I can find some good info on people trying to do the exact same thing on Gentoo. (Didn't look for Sabayon in particular, as I suspect the user base of Gentoo to be bigger and the OS itself to be better documented.)
Here's a script someone uses to set up his masquerade. You'll possibly need to modify some parameters. Be careful, you might want to backup your iptables too before doing this!
On Gentoo-wiki, you can find instructions to apparently "Masquerade the Gentoo Way". Certainly worth a read.