The large variation in the bandgap and exciton energies reported in the literature for GaN is discussed. We compare our photoluminescence data for unstrained bulk GaN with thin GaN layers grown on sapphire and SiC, respectively. It is demonstrated that the built-in strain has a strong effect on the spectral positions of the intrinsic excitons, and consequently also on the bandgap. GaN layers on sapphire have an increased bandgap, while growth on SiC leads to a lower bandgap, compared to unstrained bulk GaN. The temperature dependence of the free excitons has been studied for all three cases, and remarkably strong differences are reported.