GNet is just a way of finding games, a bit like GameSpy. The server registers a game with a GNet server ( usually this site, but you can get the php to do your own central server if you want ) and then clients connect to the GNet server to get a list of active games. It's just a lot nicer and easier than using IP addresses and Port Numbers in text boxes.
It's entirely free to use.
It's entirely free to use.