All(automotive) shocks are hydraulic. Gas shocks use a gas charge in addition to the oil (hydraulic). The gas helps suppress the foaming that is created within the shocks during use. Foam puts bubbles in the oil and prevents the shock consistent dampening performance. So that being said I don't believe most Drag only shocks are gas shocks where as roundy round or street shocks would be. Although the drag shock does do a considerable amount of work each pass typical drag surfaces are considerably smoother than most street and oval surfaces (strokes /per minute)
If this is a Drag only application I don't believe you "Need " a gas shock