Magnetic transition metal doped gallium nitride (GaN) layers play a role for their application as diluted magnetic semiconductors (DMS) and thus for the realisation of new spintronic devices. In this work, iron was used as the transition metal in a GaN matrix. In order to collect information about the states of ironions in GaN it is necessary to do optical characterisation. This work deals with photoluminescence in the ultraviolet and infrared region and high-energy ellipsometry studies. The photoluminescence characterisation was made not only on iron doped GaN but also on magnesium, silicon doped and nominally undoped GaN layers. The evaluation of the thermal behaviour of free excitons brought information on their binding energies, the effective hole mass as well as bandgap, band splitting of the valence band and effect of strain on the band structure. From the energies of bound excitons and the thermal dependence of their intensities we were able to distinguish between donors and acceptors to which the excitons are bound. Applying Hayne´s Rule we obtained their energy levels. The investigation of donor acceptor transitions, free to bound transitions and defect luminescence bands of the films allowed us to get further information on the dopants´ levels as well as the crystalline quality.<br />Concerning the iron doped samples, we obtained an insight into the location of the charge transfer levels within the fundamental bandgap. Purpose of the high-energy ellipsometry measurements on pure GaN was to obtain data on the optical response in an energy range above the fundamental bandgap.