As long as there is some substantial non-infringing use that it offers, I don't see how anyone could consider it illegal. Napster was MP3 only and there were almost no legal files available (and they used centralized servers), so it got shut down.

The Gnutella network does have files available that are legal. Someone posted a message about sharing MP3 recordings of his class lectures. Probably 90% or more of the files on Gnutella aren't legal to download unless you own a copy of the original. But since it's not centralized, it would be pretty hard to shut Gnutella down even if it were declared illegal.