gtk-gnutella is a servant on the Gnutella peer-to-peer network. In order to make the most out of gtk-gnutella it helps to understand a bit about peer to peer networking in general, and the Gnutella network in particular.
The concept of a peer to peer network is that each participant in the network has the same level of rights in the network. Everyone is equal, no one is unique, and everyone can be missed without jeopardizing the network.
To make this possible each participant has the full functionality needed to create the network. Any two participants together can create a fully functional network. To realize why this is special consider how information is normally distributed on the Internet.