a friend of mine has a G4 with airport card in it that he's using as a software base station in X. There was a tip on how to do it a while back at Macfixit (for OSX).
Basically, it tells you how to write a script to forward shit from your main interface to your other one (whether it be airport or a 2nd NIC or whatever). Then you set your 2nd NIC in Network prefs to the IP of the router that the machine is. Sounds kinda weird, but if your machine is now 10.0.1.1 (most common) then set it's IP to that also, as well as a subnet of 255.255.255.0 and router of 10.0.1.1 (yes it should be the same). Basically you're setting the 2nd NIC to itself.
You also need to ipforwarding from "NO" to "YES" in /etc/hostconfig
I can't remember exactly what needs to be written into that script that has to be read (the most important part, step 1 of 3 basically). I can find out and report back here, and/or provide an URL to said script.
However, you can do what I did. Your airport card being used as a base station is just a 2nd NIC (as said above). I have 2 NIC's in my g3 tower which I use as my router. I too could not get gNAT to work.
However, I got a program called Brickhouse which sets up a firewall and/or masquerading firewall/nat (ip sharing). Using this, I had a router up and running in about 3 minutes. The G3's built in ethernet connected to the cable modem, and the 2nd NIC connected to my switch (or a hub if that's what you want). Works flawlessly and is very fast.
While I am trying to get this NAT script for you guys, give Brickhouse a try. By default, it calls the 2nd NIC the airport card, so that's what it's ipsharing functionality is geared towards. There is excellent documentation as well, but all you should have to do is open up Brickhouse setup your info, tell it to start IPsharing on the airport card, and save. To be safe, also go to the Network prefs and set the airport cards info to itself, the IP being what you specified in in Brickhouse.
GIve it a try, i think you'll like it, and having a GUI to do all this, rather than mucking around in unix will be nicer.
you guys can IM me also if you need faster response and such (vasutu).