G2 is a totally different network, started and formed only by Shareaza. There are some clients that were going to support it, but I don't know wether they do or not.
Ultrapeers is a different thing. Until last year, all Gnutella servers where the same. Now, there are two (or three, counting the old normal nodes) kinds: ultrapeers and ultraleaves. Ultrapeers are generally people who have a fast connection and a lot of RAM, so that people with slow connections can become ultraleaves and use as less bandwith as possible. Yes, they are part of Gnutella. There is a very ilustrative picture of how it works at limewire.com
Ivan
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