We report a comparative study by luminescence and reflectivity of GaN grown on sapphire by MOVPE, HVPE and GSMBE. Whatever the growth technique, undoped GaN shows well resolved reflectivity spectra, allowing hetero-epitaxial strain to be taken into account in the interpretation of luminescence spectra. MOVPE provides the highest purity samples so far. From the luminescence study of n- and p-type doped samples, activation energies of 265 +/- 10 meV and 34 +/- 2 meV are deduced for Mg accepters and Si donors respectively. From the luminescence spectra of HVPE grown samples, the presence of a perturbed interfacial n(+) layer is evidenced.