I see that on KDE Neon which is Linux distribution based on Ubuntu LTS, gtk-gnutella is not available in official package repository.
But it worked to download the .deb package manually from
https://sourceforge.net/projects/gtk-gnutella/files/
This is working download + installation command for Debian based distributions:
Code:
wget -q -c https://sourceforge.net/projects/gtk-gnutella/files/gtk-gnutella/1.2.3/gtk-gnutella_1.2.3-1_amd64.deb/download && sudo dpkg -i download && rm -f download
before running it, use above link to check if 1.2.3 is really latest available version