A gas cylinder actuator with overtravel indicator device, which includes a tubular containment jacket, an end face, an opposite annular guiding portion, a stem-piston arranged so as to pass through the annular guiding portion, wherein the stem-piston protrudes from the jacket in an axial direction, a chamber for pressurized gas being defined between the tubular jacket, the end face, the annular guiding portion for the stem-piston, an indicator device for indicating that overtravel for the stem-piston has occurred; and an annular body, which is inserted partially into a corresponding seat between the jacket and an annular portion so as to surround the outer lateral surface of an end portion of the annular guiding portion; the annular body rests in an axial direction on an element that protrudes radially within the jacket and is integral with the jacket.