Background: a firewall in integrated in (almost) every linux system: what actually takes care of rejecting and/or forwarding is iptables, which mostly resides in kernel. However, iptables config is pretty hard to understand. Have a look by 'typing iptables --list'.
Think of it as a list of which streets are open or blocked, and you need to allow certain points to be connected. Unless you know really well the city (your network) and spend much time thinking at this, you would likely open the main avenues. This is quite unsafe, since burglers, cia agents and even tanks could get stright to your city core.
That's where 'firewalls' come in: they ease you the task to only allow wanted folks in your town (sounds xenophobist, does't it? ;p )
Back to the point:
I actually use shorewall as a firewall, not gshield, but i suppose gshield too has a config file/dir under /etc: how did you setup the firewall?
For example, in my shorewall, I would have pu this line in /etc/shorewall/rules:
# #ACTION SOURCE DEST PROTO DEST SOURCE ORIGINAL
# # PORT PORT(S) DEST
# DNAT net loc:192.168.1.8 tcp 8013
So please look for a /etc/gshield or similar and post it