JTella is horribly outdated, so it can be used for general p2p demonstration, but not for a demonstration for gnutella (the changes since then in easily readeable format:
Main Page - Gnufu - Gnutella for Users).
But you can simply use Phex (it's licensed under the GPL (the major free software license:
Philosophy of the GNU Project - GNU Project - Free Software Foundation (FSF)
)
If you need help, please don't hesitate to ask.
Best wishes,
Arne