This thread might contain your solution:
First, I needed to use su to switch to user "gdm":
sudo su - gdm -s /bin/bash
(If running in a non-GUI context, such as ssh, I also needed to run :
export $(dbus-launch) )
Then I was able to set some gnome settings as the the user "gdm"
GSETTINGS_BACKEND=dconf gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type suspend
GSETTINGS_BACKEND=dconf gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-ac-timeout 120
This sets the GDM3 screen to suspend after 2 minutes idle. Verify the
settings with "get" commands:
GSETTINGS_BACKEND=dconf gsettings get org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type
GSETTINGS_BACKEND=dconf gsettings get org.gnome.settings-daemon.plugins.power sleep-inactive-ac-timeout
These values can also be modified with GUI dconf-editor. Before the su
command, you need to run (as your regular user)
and then after the change to gdm user via su, you can run:
Be careful, though, with using this app.