Most distros will have the rules in a single script, they are really easy to read, modify and understand. I don't understand what good a GUI would do for something as simple and important as a rule-based firewall, GUIs only hides things.
Of the top of my head:
iptables -A INPUT -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A INPUT -p 22 -j ACCEPT
iptables -P INPUT -j DROP
iptables -P OUTPUT -j ACCEPT
iptables -P FORWARD -j DROP
To get a detailed overview of the rules:
iptables -nLv
If you need any simpler, just go with the defaults your distro has to offer, they'll be secure.