Gnutella doesn't have a central server, you're connecting to other computers

If you've got UPnP enabled within the router, do you also have it checked within LW's tools > options > advanced > firewall config?
From what some of the forum gurus say, I gather that some Linksys models are a bit unreliable in UPnP mode. You could try port forwarding, but to give you detailed instructions, we're going to need the model number. Have a look all over the router, it should be written somewhere. If there's a manual floating around somewhere, it should be written there as well.