gentoo iptables rules survive reboot
From thelinuxwiki
gentoo will automatically load rules from when starting iptables...
/var/lib/iptables/rules-save
the file above is saved by running...
/etc/init.d/iptables save
So, as long as iptables is in your default run level for automatic startup, and you rules are saved, they will survive a reboot. Don't forget about ipforwarding too.
fwbuilder
When using fwbuilder to install rules to gentoo, the easiest way for the rules to survive reboot is to...
Firewall object -> Firewall Settings... -> Prolog/Epilog ->
add "/etc/init.d/iptabels save" to Epilog (bottom) window under the header "The following commands will be added veratim after generated configuration"