jonaskellens
There is no seperate firewall specific GUI and it is recommended that you do not install any additional GUI control panels.
Firewall control is integrated into server manager GUI.
As you enable & disable services etc, then appropriate ports are opened or closed.
You can do more specific control with db commands and custom templates using the tips indicated in the Firewall FAQ
http://wiki.contribs.org/SME_Server:Documentation:FAQ#FirewallPlease understand that the Firewall functionality is tightly integrated into the design of sme server and if you "wipe out" all firewall iptables rules you are likely to create a serious mess and an insecure server unless you really really know what you are doing.
It will be much better for you to learn to use sme server "as is" and limit your manipulation of the firewall to the tips shown in the FAQ.
Note there is also a port opening and forwarding panel in server manager if you have specific requirements for port management.
For more sophisticated control of the firewall, you will be better off putting a seperate firewall in front of sme server and changing sme to server only mode.