The novel deposition technique of Ga2O3(Gd2O3) film by using in-situ molecular beam epitaxy (MBE) has led to the first demonstration of enhancement mode GaAs metal oxide semiconductor field effect transistors. For sub- micron GaAs device applications, the current leakage in the gate dielectric of reduced thickness has been an important issue. In this work, we address this aspect for the Ga2O3(Gd2O3) thin films deposited on n-type GaAs and present the electrical characteristics of the GaAs MOS structures as a function of the gate dielectric thickness, varying from 16.6 nm to 7.7 nm. The as-deposited thin dielectric layers show, in dark an inversion layer formation as well as an excellent insulator performance: a gate leakage current density as low as 10-9 A/cm2 at low gate bias up to 2.5 V and the electrical breakdown field reaches above 10 MV/cm. The high resolution transmission electron microscopy measurements show a sharp and uniform dielectric/GaAs transition with interfacial roughness < 1 nm.