Positron annihilation spectroscopy, supported by ab initio theory, has been applied to verify the decoration of Ga vacancies in GaN by oxygen and hydrogen. Our results indicate that the Doppler broadening measurement of electron momentum distribution is sensitive enough to distinguish even between N and O atoms neighboring the Ga vacancy. We identify isolated VGa in electron irradiated GaN and VGa-ON complexes in highly O-doped high-purity GaN. Evidence of H decoration of Ga vacancies is obtained in epitaxial GaN grown by metalorganic chemical-vapor deposition.