How to setup a development network for the ENC28J60 on XP?
The gateway is a just a default network node used to find addresses not in the routing table. Wikipedia has quite a good article that explains it here:

Default gateway

But in your case for a start I'd set the computer to have a fixed IP address of 192.168.0.1 which should work, later you can use 'route print' to see how XP sets up the routing table for that device, but it may not appear depending on other settings. You can setup mutiple IP addresses per adapter so your 'other stuff' like the Internet will still work.