The high-pressure wurtzite to rocksalt phase transition of high quality epitaxial GaN has been studied using Raman scattering and optical absorption spectroscopy. The results are compared with ab initio calculations performed within the framework of density functional theory. The Raman scattering due to the phonon modes of the wurtzite phase was followed up to 69 GPa; the phase transition was marked by the appearance of new features which we assign to disorder activated, symmetry forbidden, scattering from the rocksalt phase. There is no evidence for the recently proposed metallic MgO-h intermediate phase. Absorption measurements show an absorption feature at 1.7 eV for the rocksalt phase at 69 GPa.