If you want to suspen your gnome session when it is idle at the login screen use the following procedure.
Add gnome-power-preferences to GDM's autostart directory
Open terminal and run the following command
sudo ln -s /usr/share/applications/gnome-power-preferences.desktop /usr/share/gdm/autostart/LoginWindow/
Now you need to Log out. The gnome-power-preferences window should show up on the GDM screen.
You can Configure it how you want, then close the window and log back in.
When you're done and want the window to stop opening with GDM, run the following command from your terminal
sudo unlink /usr/share/gdm/autostart/LoginWindow/gnome-power-preferences.desktop
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