Added correct path for Gentoo iptables config file.
Gentoo also have a service to enforce iptables rules, maybe you can consider to handle it in some way.
I would have done that by myself, but this module looks to be still under heavy development and I don't want to create new issues.
Merge pull request #5214 from faust/iptables-gentoo

Added correct path for Gentoo iptables config file.