An apparatus for supplying a fuel gas (G, G') from a fuel gas source to a gas engine, including: a zero pressure regulator, a Venturi mixer, a mixture supply duct, a valve, and a controller; the Venturi mixer being a gap mixer and including a flowpath and a first fuel gas inlet; the flowpath having an air inlet region and a mixing region downstream of the air inlet region, the flowpath reducing in section area from the air inlet region to the mixing region; the first fuel gas inlet opening into the mixing region of the flowpath; the zero pressure regulator being arranged to supply a fuel gas from the fuel gas source to the first fuel gas inlet at a first static pressure substantially equal to a static pressure of air supplied to the air inlet region of the flowpath; the mixing region being arranged to mix the fuel gas flowing from the first fuel gas inlet with the air flowing from the air inlet region of the flowpath to form a mixture of fuel gas and air; the mixture supply duct being arranged to receive the mixture of fuel gas and air flowing from the mixing region of the flowpath; wherein a second fuel gas inlet is arranged to open into the mixture supply duct downstream of the mixing region of the flowpath, and the valve is controllable to supply fuel gas (G') from the fuel gas source at a second static pressure higher than the first static pressure, via the second fuel gas inlet, to the mixture of fuel gas and air flowing from the mixing region of the flowpath; the controller being configured to control the valve to initiate or increase the supply of fuel gas (G') from the fuel gas source via the second fuel gas inlet, during operation of the engine, responsive to an increased load condition of the engine; characterised in that the Venturi mixer is arranged to form the mixture of fuel gas and air flowing from the mixing region of the flowpath with an air:fuel gas ratio of not more than 2.3:1 by volume; and the flowpath in the mixing region is substantially annular, and the first fuel gas inlet comprises a substantially annular aperture, the aperture having a variable length in an axial direction of the flowpath, the length of the aperture being controllable to vary an air:fuel gas ratio of the mixture; and the controller is configured to control the length of the aperture to vary an air:fuel gas ratio of the mixture flowing from the mixing region of the flowpath, and, during operation of the engine, to initiate the supply of fuel gas (G') via the second fuel gas inlet only in a maximally open condition of the first fuel gas inlet. a zero pressure regulator, a Venturi mixer, a mixture supply duct, a valve, and a controller; the Venturi mixer being a gap mixer and including a flowpath and a first fuel gas inlet; the flowpath having an air inlet region and a mixing region downstream of the air inlet region, the flowpath reducing in section area from the air inlet region to the mixing region; the first fuel gas inlet opening into the mixing region of the flowpath; the zero pressure regulator being arranged to supply a fuel gas from the fuel gas source to the first fuel gas inlet at a first static pressure substantially equal to a static pressure of air supplied to the air inlet region of the flowpath; the mixing region being arranged to mix the fuel gas flowing from the first fuel gas inlet with the air flowing from the air inlet region of the flowpath to form a mixture of fuel gas and air; the mixture supply duct being arranged to receive the mixture of fuel gas and air flowing from the mixing region of the flowpath; wherein a second fuel gas inlet is arranged to open into the mixture supply duct downstream of the mixing region of the flowpath, and the valve is controllable to supply fuel gas (G') from the fuel gas source at a second static pressure higher than the first static pressure, via the second fuel gas inlet, to the mixture of fuel gas and air flowing from the mixing region of the flowpath; the controller being configured to control the valve to initiate or increase the supply of fuel gas (G') from the fuel gas source via the second fuel gas inlet, during operation of the engine, responsive to an increased load condition of the engine; characterised in that the Venturi mixer is arranged to form the mixture of fuel gas and air flowing from the mixing region of the flowpath with an air:fuel gas ratio of not more than 2.3:1 by volume; and the flowpath in the mixing region is substantially annular, and the first fuel gas inlet comprises a substantially annular aperture, the aperture having a variable length in an axial direction of the flowpath, the length of the aperture being controllable to vary an air:fuel gas ratio of the mixture; and the controller is configured to control the length of the aperture to vary an air:fuel gas ratio of the mixture flowing from the mixing region of the flowpath, and, during operation of the engine, to initiate the supply of fuel gas (G') via the second fuel gas inlet only in a maximally open condition of the first fuel gas inlet.