A system for supporting and protecting at least one gas tank of a gas fuel system on-board of a motor-vehicle, wherein said gas tank is arranged to protrude below a floor of a motor-vehicle and said system is adapted to be associated with a floor of a motor-vehicle at a gas tank, said system comprising a covering element (4, 5) rigidly connectable below the floor of the motor-vehicle, wherein said covering element (4, 5) is cup-shaped to cover the outer body of said gas tank and, when said system is associated with the floor of the motor-vehicle at a gas tank, has an upper surface facing said gas tank and a lower surface facing the ground, so that said covering element (4, 5) protects said gas tank protruding below the floor of the motor-vehicle from interference with foreign bodies, when said system is associated with the floor of the motor-vehicle at a gas tank, said covering element is in direct or indirect contact with said gas tank, and said gas tank is supported by the floor of the motor-vehicle by means of said covering element, without the aid of additional elements to support said gas tank, said covering element is adapted to be connected below the floor of the motor-vehicle by means of a plurality of fastening elements arranged along a perimeter flange of said covering element (4,5), said system also comprises at least one locking band intended for partially wrapping around a gas tank, wherein said locking band is rigidly connected to the upper surface of said covering element at its end portions in such a way that, when said system is associated with the floor of a motor-vehicle at a gas tank, said locking band secures the gas tank within the covering element so as to avoid movements of the gas tank caused by the movement of the motor-vehicle, said system being characterized in that: said covering element has a plurality of reinforcing brackets, which are arranged to give greater rigidity to the covering element at points of maximum stress due to the weight of the gas tank, said brackets being rigidly connected to a central portion of the covering element at its lower surface by further fastening elements, said fastening elements arranged along the perimeter flange of said covering element (4,5), being also used, at least in part, to attach said brackets to the covering element, and said covering element has at least one band element of polymeric elastic material at its central portion arranged on the upper surface of said covering element, in such a way that, when said system is associated with the floor of the motor-vehicle at a gas tank, the contact between said gas tank and said covering element is achieved by means of said band element of polymeric elastic material. said covering element has a plurality of reinforcing brackets, which are arranged to give greater rigidity to the covering element at points of maximum stress due to the weight of the gas tank, said brackets being rigidly connected to a central portion of the covering element at its lower surface by further fastening elements, said fastening elements arranged along the perimeter flange of said covering element (4,5), being also used, at least in part, to attach said brackets to the covering element, and said covering element has at least one band element of polymeric elastic material at its central portion arranged on the upper surface of said covering element, in such a way that, when said system is associated with the floor of the motor-vehicle at a gas tank, the contact between said gas tank and said covering element is achieved by means of said band element of polymeric elastic material.