[gnome] How to register an application in the system ?

Can you locate the .desktop file for gnome-font-viewer? It should be in /usr/share/applications. Copy that file over to ~/.local/share/applications without using sudo.

Then look for the line starting with Exec=. Does it end with %f? If it doesn't, just add that at the very end of the Exec= line. In other words, add a simple space followed by %f.

Now see if Nautilus offers gnome-font-viewer as an option in Open With ....

(Idea from https://ubuntugenius.wordpress.com/2...s-in-nautilus/)

If it doesn't, no big deal. You still have your original .desktop file in /usr/share/applications.

BTW, I think in technical terms, anything installed using dpkg (and that means via the software center or apt or apt-get or gdebi) is registered with the system automatically. There's nothing further that needs to be done.