Hexagonal a-plane GaN films with (1120)-orientation were grown by metalorganic vapour phase epitaxy on r-plane sapphire substrates. Spectroscopic ellipsometry in the photon energy range from 1.2 up to 5 eV was applied in order to determine the ordinary and extraordinary complex dielectric function of GaN. A distinct optical anisotropy is found over the whole energy range which is emphasized by reflectance anisotropy studies. The polarization dependent shift of the absorption edges is confirmed by photoreflectance measurements and compared to band structure calculations for which the in-plane strain is taken into account.