The version of Gtk-gnutella in the Ubuntu-Trusty repo is 1.0.1-1. That version might be able to work on the network still. I would suggest updating to a newer version of Ubuntu (like maybe Ubuntu-Wily). Ubuntu-Xenial has Gtk-gnutella 1.8.1-1, which is pretty new.

If you want to stay with Ubuntu-trusty you might have to brave and learn how to compile Gtk-gnutella yourself.

Or you could try downloading and installing it from sourceforge: You'll have to do something like and hope you have all the dependencies needed (I don't recommended this). If this is the version you installed, and it didn't work, you may need to use a new release of Ubuntu to get it working anyway.

If nothing works and you still wanna stay with Ubuntu-Trusty, but don't want to compile Gtk-Gnutella for youself, you can download the Windows installer for Gtk-gnutella and run it through Wine. You'll have to install Wine, but I think Gtk-gnutella works pretty good with it. Wine can be installed via any Ubuntu package manager.

If you hate using Wine you can try and see if a newer (working) version of Gtk-gnutella is available in the Ubuntu Backports for Trusty.