We present a detailed photoluminescence excitation study of the optical transitions in GaN. This technique is employed to distinguish between band-to-band excitation and exciton contribution to the formation of the free exciton, bound exciton, violet and yellow photoluminescence bands. We show the dominant role of the Fröhlich polar intraband scattering in the formation of the free exciton states. We demonstrate that bound exciton states in a large extent are created by the capture of the free excitons by shallow impurities as well as by phononassisted resonant excitation of the bound exciton states. The capture of the free carriers excited in the band continuum is a main excitation source for the violet and yellow bands. However, distinct A- and C-exciton resonances are detected in the excitation spectra of the violet and yellow emission bands.