OK, this is what I found:
Port forwarding is OK (I forwarded port 26 of the gateway to the 23 of the client and was able to telnet the client from other machines)
On the linux gateway I do, after setting up iptables with the rc.firewall script, that I got from a HOW-TO:
/sbin/iptables -A FORWARD -i eth1 -o eth0 -p tcp --dport 6001 -j ACCEPT
/sbin/iptables -A PREROUTING -t nat -p tcp -d linux_gateway_ip --dport 6001 -j DNAT --to 192.168.0.3:6001
On the client machine I do a:
xhost +
On the DIGITAL computer I do a:
DISPLAY=linux_gateway_ip:1.0
export DISPLAY
emacs &
The emacs proccess lasts a minute or so, and I guess it is trying to connect without success... Then it displays a error message:
"Error: Can open display: ..."
Since port forwarding works (I tested it with telnet), and the proccess doesn't die immediatly, I think that I'm missing something....