You are correct when you say that two p2p clients cannot connect to each other if BOTH are behind firewalls. There are a couple of things you can do to allow yourself access even when you are behind a firewall (and personaly I would never work on a computer that was not behind some sort of firewall).

1) If the firewall is local (i.e. Run from one of your computers) and not the ISP's then all you should need to do is open one port (ex. 6346 or 6347) and force your gnutella client to use a specific port (how to do this varies by firewall type and gnutella client)
*Note: If you are using Windows XP's built in firewall... I would recommend replacing it with something like Tiny Personal Firewall (do a google search, its free).

2) If the firewall is located on your ISP's router, try and find out what ports are open (some of the common ones would probaly be left open). Force your gnutella client to use a port that IS NOT USED BY ANY OTHER PROGRAM. You may have to try a few different ones until you find one that seems to work.

You can get more information on common port usage here:
<a href="http://www.chebucto.ns.ca/~rakerman/port-table.html">http://www.chebucto.ns.ca/~rakerman/port-table.html</a>

I have used the AIM (AOL Instant Messanger) port 5190 with sucess from school but I knew that port was open on the university server.

Good Luck... It will just take alot of trial and error