Ga2O3(Gd2O3), electron beam evaporated from a single crystal Ga5Gd3O12 garnet, was en: situ deposited on molecular beam epitaxy grown GaN of Ga-polar surface. Using capacitance-voltage measurement, accumulation and depletion behavior was observed in the Ga2O3(Gd2O3)/GaN metal-oxide-semiconductor diodes, with an interfacial density of states less than 10(11) cm(-2) eV(-1) The Ga2O3(Gd2O3)/GaN interface remains intact with the samples subject to rapid-thermal annealing up to 950 degrees C, as studied from x-ray reflectivity measurements.