We have studied by photoluminescence (PL) and optically detected magnetic resonance (ODMR) un-doped, n-doped and p-doped thin wurtzite GaN layers grown by metal-organic chemical vapor deposition on sapphire substrates. From the PL data for free excitons an accurate value of the free A-type exciton binding energy and a more accurate estimate for the hole effective mass is deduced. The localization energies of the Mg and the Zn neutral acceptor bound excitons are found to be in good agreement with Haynes’ rule. A sharp emission line, assigned to free electron recombination at a 116 meV shallow acceptor, together with three additional weak zero-phonon-lines (ZPLs), assigned to distant donor-acceptor (DA) pairs, are reported for the first time. The chemical nature of this acceptor and that of three residual donors, inferred from the DA pair ZPLs, is discussed. The effects of strain in thin GaN layers on a dissimilar substrate like sapphire are emphasized with respect to the energetic position of narrow PL lines. The ODMR data obtained for undoped, Mg-doped and Zn-doped GaN layers provide insight into the recombination mechanisms responsible for the broad yellow (2.25 eV), the violet (3.15 eV) and the blue (2.8 eV) PL bands, respectively. The ODMR results for Mg and Zn also show that these acceptors do not behave effective mass like and indicate that the acceptor hole is mainly localized in the nearest neighbor shell surrounding the acceptor core.