gtk-gnutella has sort of a proxy support, but it doesn't work very smoothly, if you ask me.

Port 6346 is your listening port. That's were other clients connect, when they want to open a gnutella 0.6/0.4 connection or when they want to download any files. But that's also the port where other firewalled hosts would connect to you when you want to download a file from them.

If you close your client's listening port you can't download from firewalled hosts.