Very true tp2p. As far as beginning with Gnutella, I recommend implementing standard v0.6 Gnutella protocol. It consists of everything v0.4 has, except the handshake method, which was extended to allow more features. Whether you provide backward compatibility with the v0.4 handshake is up to you, but v0.6 is pretty much considered "de facto" now.

Once you've done that, I highly recommend implementing the Ping/Pong cache sheme. Which one you choose is pretty much up to you, as all the proposed variants have the same "external effect", just work differently internally (code wise).

After that, I'd say that it will be up to you to choose which features you'd like to include, such as Ultrapeers, HUGE, compression, etc. They are not mandatory features, although they can be very beneficial to your end users.

As far the GDF goes, yes, developers there do have a tendency to drag topics a beyond comprehension. So it's hard for newcomers to follow what exactly is going on. But it's fine to jump in and ask a question, even if the same thing has been discussed before. Plenty out there that are willing to help you out.