A gas spring with a piston rod assembly received for movement between retracted and extended positions in a casing defining a main gas chamber. The piston rod assembly when in its extended position cooperates with a portion of its retainer and the casing to at least in part provide a secondary gas chamber separate from the main gas chamber and having gas trapped therein at a higher pressure than that of gas in the main gas chamber to reduce the force required to initially move the piston rod assembly from its fully extended position toward its retracted position.