What you need is assign the PC that uses Gnutella, or a server kind of application, a static IP address (say, 192.168.1.5). Then, in your firewall, forward the ports used by any such applications to that IP address. How this is done, depends on your firewall type, so you'll have to refer to the manuals for that. But I know that for Linksys routers, this can be set in the "Advanced" section.