I think you may have a different interpretation of what an internet gateway is.
A gateway is a computer that is SERVING host computers...almost like a huge firewall server would. Let's assume that your Windows 2000 computer will operate as a gateway...we'll designate it COMPUTER A.
COMPUTER A will need to have 2 network cards to operate as a gateway...whether wireless or other. The first network card would be connected to your outside line...your modem...and would be exposed to the outside world. Obviously you don't want this one to serve your LAN its access because then people would be able to get at all you computers. This is why you would put another network card in and MANUALLY ASSIGN it an internet protocol (IP) address. When you manually assign it an address, you're giving the card a way to be different from you outside line.
You second card will be a beacon to your second computer (COMPUTER B). By plugging in COMPUTER B (or wirelessly detecting) you'll be able to see the manually assigned IP address given to the 2nd NIC in COMPUTER A. Now open up your network configurations for your COMPUTER B and look for "Gateway"
Enter in the manual IP address of your second Network Card on COMPUTER A into the blank for "Gateway". Ensure that your subnet is the same on both computers. Ensure that both computers are on the same workgroup. Type in a manual IP adress for COMPUTER B (usually 192.168.0.X where X is a number between 1 and 254). Reboot and you're set..
If you are unsure about any steps above, please post. We'll step you through it.