If some program wants to connect to an other prog via internet. it needs to know the right ip-Adress to reach the mashine. Then it needs the right "port"-number to reach the right program on the mashine.

Your IP 192.168.x.x is not reachable via internet. The IP-adress you are reachable with via internet is the one of your firewall.

the port-number which tells your computer to deliver the ip-packages to the gnutella-software is probably blocked by your firewall. So first you need to allow connections on this port (usually 6346) at your firewall.
The Firewall should in no way run any software like a gnutella-client! what u need to do next is to tell the firewall, to send all ip-packages which want to establish a connection on 6346 to your qtella-computer (192.168.122.155) on your lan.
how u do this depends on what progs u use on ur firewall.

Telnet can connect to any port. i.e. you can "telnet www.gnutellaforums.com 80" to see the answer of this webserver. if u see any answer (other than "not reachable" i.e. :-]) u know that it's possible to reach a prog which listens on this port.