NOVELTY - The system comprises a gas generator unit with: (a) an anode comprising an electrochemically oxidizable substance, a hydrogen-generating electrode as a cathode and a separator arranged between the anode and cathode; and (b) a cathode comprising an electrochemically reducible substance, an oxygen-generating electrode as an anode and a separator arranged between the anode and cathode. A gas-tight reservoir is arranged in the gas generator unit for storing hydrogen or oxygen gas generated by the generator unit under pressure.    USE - The system in the form of an electrochemical cell is useful for producing hydrogen or oxygen gas (all claimed).    ADVANTAGE - The system produces large amounts of gas within few seconds from start-up of the system.    DETAILED DESCRIPTION - The system comprises a gas generator unit with: (a) an anode comprising an electrochemically oxidizable substance, a hydrogen-generating electrode as a cathode and a separator arranged between the anode and cathode; and (b) a cathode comprising an electrochemically reducible substance, an oxygen-generating electrode as an anode and a separator arranged between the anode and cathode. A gas-tight reservoir is arranged in the gas generator unit for storing hydrogen or oxygen gas generated by the generator unit under pressure. The reservoir is filled with hydrogen or oxygen at atmospheric pressure. The gas generator unit is an electrochemical cell (100), which comprises a housing enclosing an interior space in which anode, cathode, separator and optionally additional functional parts of the cell are arranged. The interior space has an internal volume, which exceeds the total volume of (a) or (b) and the optionally contained functional parts arranged in inner space around the factor of 1.1. The ratio between the inner volume and the total volume is 2-100. The housing of the cell comprises an opening through which hydrogen or oxygen gas from the reservoir is discharged into the environment. The housing of the cell comprises a cell cup (101) and a cell cover (103). The cell is designed as a button cell. The interior of the housing is divided by a partition into a first compartment (106) and a second compartment (110). The first compartment comprises the anode having the electrochemically oxidizable substance or the cathode comprising the electrochemically reducible substance. The second compartment comprises the opening of the housing. The opening of the housing is closed by a thin polymer or a metal layer, which prevents oxygen or hydrogen from escaping from the reservoir. The polymer or metal layer is formed inside the housing in a form of a film covering. The opening has a minimum diameter of less than 1.5 mm. The polymer or metal layer has a thickness of less than 40 microns. The reservoir additionally contains a flowable medium, which is discharged by hydrogen or oxygen from the reservoir.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross-sectional view of an electrochemical cell.    Electrochemical cell (100)    Cell cup (101)    Cell cover (103)    First compartment of cell (106)    Second compartment of the cell. (110)