You can get C source at gtk-gnutella: htp://gtk-gnutella.sf.net
(I can't comment on the "language A is better than B, because I don't know your definition of value
xkcd - A webcomic of romance, sarcasm, math, and language - By Randall Munroe )
When everything else is equal, I'd always choose gtk-gnutella over Gncleus, because it's the far more advanced client (and I know it to work very well).