See the bottom of the README file, and the home page mentions it too.

gtk-gnutella has now integrated some of the improvements from napshare, but was stale for over a year, so napshare was started.

Nice thing about open source, no one is "stealing" from anyone else, just improving it.

You could say he interface of gtk-gnutella is a rip-off of the original gnutella client.

So who's the original rip off now?

Chill Dude. Run whatever you are happy with, write and contribute some code of your own. No one gets paid for developing these clients for you.