A gear actuator for engaging and disengaging gears in an automatic manual transmission has a single working piston or two working pistons provided with a first working piston surface and a second working piston surface. An auxiliary piston is provided also. A pressure control valve controls a pressure supplied to the second working piston surface. A shut-off valve is provided. The first working piston surface and the pressure valve are supplied with a system pressure of a hydraulic system. The first working piston surface and the pressure control valve can be decoupled from the system pressure of the hydraulic system by the shut-off valve.