In Gnucleus, "GnucleusDoc.cpp" at about line 115
ModeNetwork = "GNUTELLA";

All you have to do is change it to
ModeNetwork = "OPENSOURCE";

And you are ready to go!
Anyone want to supply a downloadable version?
It wouldn't be hard to add to "Preferences.cpp" a line that would allow this to be changed in the "GnuConfig.ini" file.