hmm, confounding.

The other thought I had was that if you're connecting through a router it could have an internal firewall that could be fouling things up. It could be an ISP issue for that matter.

If you haven't tried it yet you could switch the port to some place else and see if that would do it. Could the chain be picky about wanting the specific IP of the interface in -d $OUTERNET rather than a network range? I don't think so, but it was another thought I had.

Since you're using IPCHAINS I'm assuming that you're running a 2.2.x kernel. It might be possible that switching to a 2.4.x kernel and netfilter could fix the problem.

I have one last question about your network setup. Are you running gtk-gnut on the proxy/firewall with direct access to the internet connection or is it running on an internal network machine?

I'm not sure any of this helped, but hopefully it will get a little closer.