Ga-doped ZnO films have been grown on GaN templates by plasma-assisted molecular beam epitaxy and characterized in terms of structural, electrical, and optical properties. In-situ post-growth annealing under Zn overpressure can effectively reduce the resistivity of the GZO films by minimizing the concentration of acceptor-like Zn vacancy caused by thermal desorption of Zn atoms. A minimal resistivity of 9.2×10−5 Ω cm A 804598 achieved on the GZO films grown at 300 °C. The metal–semiconductor transition temperature is dependent on the dopant concentration in the GZO films and related to the increased disorder induced by Ga atoms. Optical transmittance over 95% in the visible range is obtained on the films with electron concentration on the order near 1021 cm−3. By minimizing the acceptor-like defects, we have successfully demonstrated GZO films with compensation ratio near 0.4 and resistivity below 1×10−4 Ω cm, simultaneously.