We report on the fabrication and testing of GaN resistive gas sensors for hydrogen detection. The Si-doped n-type GaN was grown by organometallic vapor phase epitaxy (OMVPE) on c-plane sapphire substrates. The device structure is simply a pair of metal ohmic contact pads. The sensors are sensitive to H2 gas over a wide range of concentration: the lowest concentration tested being ~0.1% H2 (in Ar), well below the lower combustion limit in air. No saturation of the signal is observed up to 100% H2 flow. In the continuous operation mode with varying H2 concentration, a clear and sharp response was recorded with no memory effects during ramping up and down cycles of H2 concentration. The change in current at a fixed voltage to hydrogen was found to change with sensor geometry. The possible gas sensing mechanisms are still under investigation.