According to an illustrative example of the invention, a gas spring is combined with a telescopic tube which is axially slidable on the cylinder of the gas spring. The piston rod and the telescopic tube are provided at the respective most remote ends thereof with fastening eyes. The telescopic tube can be fixed in a plurality of positions with respect to the cylinder. For such fixation, the cylinder is connected with a carrier within the telescopic tube between opposite end walls of the cylinder and the telescopic tube. A locking bolt is guided within the carrier in a direction transverse to the axis of the telescopic tube. This locking bolt is engageable with a corresponding plurality of openings of the telescopic tube. The locking bolt is biased by a spring into an engagement position with respect to a respective opening and can be released from such engagement by being inwardly pressed by an operator.