General purpose output, most MO displays have a number of GPO's on them. In most cases these will supply 5v which you can use to power LED's or circuits with. Some of the MO displays will supply 12v via the GPO giving you the option to turn on and off fans and other PC equipment if wanted.
General Purpose Outlet, or power point, capable of supplying normal mains power.
General Purpose Output. The contact closure output of a General Purpose Interface