Photoreflectance (PR) spectra of high-purity, nearly free-standing GaN substrate were compared with emission and reflectance spectra, which were analyzed based on a model exciton-polariton picture in which A, B, and C free excitons couple simultaneously to an electromagnetic wave. The GaN substrate with reduced dislocation density was prepared by lateral epitaxial overgrowth technique and it exhibited predominant excitonic emissions with the decay time nearly I ns even at room temperature. The transition energies obtained from the PR spectrum agree with the energies of bottlenecks of the excitonic polariton branches. The result means that perturbation-induced change in the dielectric function is mainly due to polaritons. Temperature dependence of the A-exciton energy was well described using a model which assumes Einstein phonons.