My guess(and it is a guess) is that since it's on a network, you'd have to be an administrator of the network to be able to alter the settings of the firewall to allow Gnotella access.

I'm usin Norton Internet Security, and since I'm NOT on a network, I am my own administrator so to speak, and have made a rule in my firewall to allow Gnotella access.

But as I said - if you're on a network, I think you have to be an admin to gain access to the firewall rules.