Gas cylinder, which comprises a tubular body arranged along a longitudinal axis and having a first end and a second end, and which also comprises a piston assembly and a shaft that are jointly movable in the interior of the body in the direction of the longitudinal axis, wherein the piston assembly is adjusted internally to the body and delimits a first chamber and a second chamber containing gas, where the piston assembly also provides a gas passageway (16, 17, 18, 19) between said chambers (10, 11), and wherein the shaft is arranged through the second chamber and protrudes from the second end of the body, the gas cylinder comprising: - a tubular piece arranged in the interior of the body and in the area of the second end of the body, wherein the tubular piece is crossed through by the shaft leaving an interior cavity between the tubular piece and the shaft, wherein the interior cavity ends in an opening that communicates the interior cavity with the second chamber; wherein - the cylinder comprises a neck (33; 40; 50) adjacent to the piston assembly and that widens the shaft, whereby this neck (33; 40; 50) is sized to penetrate inside the interior cavity, characterised in that the cylinder comprises a plurality of additional spaces, in addition to the interior cavity, communicated with the interior cavity and therefore capable of storing gas at the same pressure as in the interior cavity, wherein said plurality of additional spaces comprises: - an exterior cavity, delimited between an exterior recess of the tubular piece and the body, - an intermediate cavity, delimited between the tubular piece and a fixed assembly located at the second end of the body, wherein the cylinder comprises a gas through space between the intermediate cavity and the exterior cavity, and wherein - at least one airtight elastic element (34; 42; 52) is placed on an end of the neck (33; 40; 50) closest to the second end of the body, wherein the airtight elastic element (34; 42; 52) provides an airtight closure in the interior cavity and in the additional spaces when the end of the neck (33; 40; 50) penetrates within the interior cavity. - a tubular piece arranged in the interior of the body and in the area of the second end of the body, wherein the tubular piece is crossed through by the shaft leaving an interior cavity between the tubular piece and the shaft, wherein the interior cavity ends in an opening that communicates the interior cavity with the second chamber; wherein - the cylinder comprises a neck (33; 40; 50) adjacent to the piston assembly and that widens the shaft, whereby this neck (33; 40; 50) is sized to penetrate inside the interior cavity, - an exterior cavity, delimited between an exterior recess of the tubular piece and the body, - an intermediate cavity, delimited between the tubular piece and a fixed assembly located at the second end of the body, wherein the cylinder comprises a gas through space between the intermediate cavity and the exterior cavity, and wherein - at least one airtight elastic element (34; 42; 52) is placed on an end of the neck (33; 40; 50) closest to the second end of the body, wherein the airtight elastic element (34; 42; 52) provides an airtight closure in the interior cavity and in the additional spaces when the end of the neck (33; 40; 50) penetrates within the interior cavity.