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GDM auto-suspend (GNOME 3.28)
GDM uses a separate dconf database to control power management. You can make GDM behave the same way as user sessions by copying the user settings to GDM's dconf database.
$ IFS=$'\n'; for x in $(sudo -u YOUR_USER gsettings list-recursively org.gnome.settings-daemon.plugins.power); do eval "sudo -u gdm dbus-launch gsettings set $x"; done; unset IFS