The valence-band offset (DeltaE(V)) has been determined to be similar to2.6 eV at the Ga2O3(Gd2O3) GaAs interface, and similar to1.1 eV at the Ga2O3(Gd2O3)-GaN interface by high-resolution X-ray photoelectron spectroscopy with synchrotron radiation beam. The Pt-Ga2O3(Gd2O3)-GaAs MOS diode exhibits a current-voltage characteristics dominated by Fowler-Nordheim tunneling. From the current-voltage data at forward and reverse biases, we have extracted a conduction-band offset (DeltaE(C)) of similar to1.4 eV at the Ga2O3(Gd2O3)-GaAs interface and an electron effective mass m* similar to 0.29m(e) of the Ga2O3(Gd2O3) layer. Consequently, the energy-band gap of Ga2O3(Gd2O3) is similar to5.4 eV, while DeltaE(C) for the Ga2O3(Gd2O3)-GaN interface is similar to0.9 eV.