Morgwen is right here.

Try out any other port in your gnutella client. Pehaps something common as port 20, 23, 79, 80 or 6666 (ports usually used for telnet, ftp, finger, http, irc - if not allready used by another server application on your computer)... or use a random highport (e.g. > 9999).

BUT this will only work if your provider is stupid and blocks incoming traffic _to_ 6346/6347. Expect (!!!) that your ISP will block incoming traffic _from_ 6346/6347. I that case you have a problem. Most gnutella servants run on these ports (according to my hostslists) ... only very less servants don't.
And even when you find one servant that does run gnutella on a gnutella-untypical port (not 6346/6347) and you connect via this servant, your chances are near zero that you can download files!
You might find files, but you will nearly never download. Because the direct P2P file transfar has a high chance to come from a 6346/6347 host, which is blocked by your ISP.
Solution... sorry no, only if you are a geek. Then you can tunnel all 6346 traffic through another host and port outside your ISP (port forwarding or socks2http or...). Okay there is an idea, you maybe wanna test other file sharing systems like eDonkey, Morpheus... visit www.zeropaid.com.

Good luck!