The composition, surface structure, and electronic structure of zinc blende-GaN films grown on GaAs (100) and (110) by plasma-assisted molecular beam epitaxy were investigated by means of core and valence level photoemission. Angle-resolved photoelectron spectra (photon energy 30-110 eV) exhibited emission from the Ga 3d and N 2s levels, as well as a clear peak structure in the valence band region. These peaks were found to shift with photon energy, indicative of direct transitions between occupied and unoccupied GaN bands. By using a free electron final band, we are able to derive the course of the bands along the Gamma-X and Gamma-K-X directions of the Brillouin zone and to determine the energy of critical points at the X point. The relative energies of the Ga 3d and nitrogen 2s bands were also studied, and a small amount of dispersion was detected in the latter. The resulting band structure is discussed in relation to existing band structure calculations.