A system (S) 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 (P) of a motor-vehicle and said system (S) is adapted to be associated with a floor (P) of a motor-vehicle at a gas tank , said system (S) comprising a covering element rigidly connectable below the floor (P) of the motor-vehicle, wherein said covering element is cup-shaped to cover the outer body of said gas tank and, when said system (S) is associated with the floor (P) of the motor-vehicle at a gas tank , has an upper surface (S1) facing said gas tank and a lower surface (S2) facing the ground, so that said covering element protects said gas tank protruding below the floor (P) of the motor-vehicle from interference with foreign bodies, when said system (S) is associated with the floor (P) 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 (P) 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 (P) of the motor-vehicle by means of a plurality of fastening elements arranged along a perimeter flange of said covering element , said system (S) 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 (S1) of said covering element at its end portions in such a way that, when said system (S) is associated with the floor (P) 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 (S) 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 (S2) by further fastening elements , said fastening elements arranged along the perimeter flange of said covering element , 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 (S1) of said covering element , in such a way that, when said system (S) is associated with the floor (P) 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 (S2) by further fastening elements , said fastening elements arranged along the perimeter flange of said covering element , 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 (S1) of said covering element , in such a way that, when said system (S) is associated with the floor (P) 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 . <EOS>