Oxidized Ga-polar GaN surfaces have been studied both experimentally and theoretically. For in situ oxidization at 550 degrees C using molecular oxygen, Auger electron spectroscopy indicates a saturation oxygen coverage of 2.1 +/- 0.5 ML (monolayer). For these surfaces scanning tunneling microscopy reveals two surface phases, one with 3 root 5 X 3 root 3 - R30 degrees periodicity and the other with disordered two times periodicity. Scanning tunneling spectroscopy revealed a surface band gap with size close to that of GaN, indicating that any states of the oxide lie predominantly outside of the GaN gap. From first-principles total energy calculations of surface formation energies two models of energetically favorable surfaces structures are developed, with oxygen coverages of 1.25 and 2 ML, respectively. Both structures satisfy electron counting and have surface band gaps close in size to that of GaN, in agreement with experiment. (c) 2006 American Vacuum Society.