Standard hydraulic shocks absorbers use air and oil to cushion the stroke of the shock absorber. A gas shock replaces the air in the shock with pressurized nitrogen gas. The nitrogen gas keeps it cooler than a hydraulic shock. Also when a hydraulic shock is used on rough surfaces (ie washboard) the hydraulic oil may foam thus reducing its effectiveness. The nitrogen in gas shocks helps to eliminate the potential for oil to foam.
An additional benefit of a gas shock absorber is that it creates a mild boost in spring rate.