I have showed you how make a user not appear at startup, in LightDM, here. In this article will show you how to hide a user, from the GDM login screen.
This is usefull for example, if you have a user created only for backups and you don’t want anyone to see it in the login screen.
This is what you have to do hide the users anakin and r2d2, from the GDM screen:
Edit the [greeter] section in the /etc/gdm/custom.conf so that it looks like this:
[greeter]
Exclude=nobody,anakin,r2d2
On the Exclude line, add the users you do not want to appear on the GDM screen. You can add how many usernames you want, separated by commas.