Hall-effect, photoluminescence (PL), cathodoluminescence (CL), and Raman scattering measurements have been used to characterize the Ga (top) and N (bottom) faces of free-standing GaN layers grown by hydride vapour phase epitaxy on Al2O3. The material near the bottom has higher carrier concentration, lower mobility, larger PL linewidths, brighter CL emission, and stronger Raman plasmon-phonon lines than the material near the top. All results are consistent with the diffusion of O from the Al2O3 substrate, sometimes covering a distance of several tens of micrometres. The O donor is compensated by Ga vacancy acceptors, known to exist from positron annihilation experiments. However, Raman and CL profiling show that the poor interface region ends rather abruptly, giving excellent material near the top (Ga) face.