Gdm custom configuration file location

If I want to create a custom gdm configuration file, where do I put it? I tried creating a directory/file of /etc/gdm/gdm.conf but that did not work. I am trying to get a multiseat system up and running. gdm does not seem to recognize my configuration (but lightdm does). So I figure it must be something in the configuration file.

I also didn’t find a way to apply the parameters in the custom.conf file. But I found an alternative, you can directly edit the /usr/share/gdm/gdm.schemas file using gconf or change some parameters in /org/gnome/login-screen using sudo -u gdm dconf-editor.

See GNOME Display Manager Reference Manual

no changes to /usr directory will be overidden by swupd