installing a package not from GNU

[Gottfried]

Hi,
I wanted to install a package "gtkhash" which I could not find in GNU 
packages.
How can I install it?
After downloading it and trying to install it, it said: unknown package
guix install gtkhash-1.4
guix install: Fehler: gtkhash-1.4: Unbekanntes Paket


What to do, if I don´t find a package in GNU and I would like to install it?


gottfried