Extinguish all naked flames and do not turn any electrical appliances or lights on or off. Open doors and windows to ventilate the room your stove is in.
Turn your gas leak detector on. As the operating instructions for different gas leak detectors will vary, you'll need to consult your unit's user guide.
Hold the tube connected your gas leak detector up to the gas outlets on your stove. You should hear a noise and see an increased reading on your detector's screen if gas is detected. Then, move over to the gas inlet valve and the source of your gas supply.
Turn off your gas supply once you've identified the source of the leak and arrange for it to be repaired.