Micro-infrared reflectivity and micro-Raman scattering have been used to determine free electron density and residual strain distribution in bulk GaN crystals. As-grown samples exhibit significant variation of electron concentration and strain along their surfaces. Increase of electron concentration correlates with the growth direction. The observed inhomogeneities in the properties of bulk GaN crystals can be eliminated by mechanical polishing of the crystal and removing the surface layer or growth figures formed during the sample cooling, which follows its high-pressure, high-temperature growth. Properties of that surface layer can differ from the bulk due to different growth mechanisms and impurity distribution. We suggest that oxygen is a likely candidate for the donor impurity supplying high amount of electrons to the conduction band of GaN.