you are talking about " lock screen" - the default is normally 5 min.
that is not the same as logging you out
the way to set that is using the gui tools for whatever desktop you are using
the directions will be different for KDE, or Gnome, or xfce , or whatever
and is NOT set in gdm
i am guessing Gnome 3
it should be in the screensaver gui
Last edited by John VV; 09-14-2013 at 06:03 PM.