The influence of biaxial stress on the optical properties of thin GaN films is studied by x-ray diffraction and Raman and photoluminescence spectroscopy. The stress is caused by differences in the thermal expansion coefficient and lattice mismatch between the film and c-plane sapphire substrates. In particular, the influence of various thicknesses of AlN buffer layers on the strain in GaN films is studied. GaN/AlN films were deposited by low pressure metal organic chemical vapor deposition using triethylgallium and tritertbutylaluminum and ammonia. We observe a pronounced reduction of strain in the GaN films with increasing buffer thickness: An AlN buffer layer thicker than 200 nm eliminates the stress completely. Estimates of the linear coefficient for the near band gap luminescence shift due to biaxial compressive strain yield a value of 24 meV/GPa.