We present a comprehensive overview of the optical properties of zinc-blende GaN. By a variety of different methods, such as temperature-dependent photoluminescence, photoluminescence excitation spectroscopy, photoreflectance, and ellipsometry, we investigate its emission and absorption related characteristics. The sample under study is a nearly strain-free epitaxial layer grown on freestanding cubic SiC. The light-hole/heavy-hole exciton was found at 3.271 eV at 5 K, shifting to 3.208 eV at 295 K. The split-off exciton transition was detected to be 21 meV higher in energy. Taking the difference in the exciton binding energies into account, this yields a spin-orbit energy of Δso=15 meV. Donor and acceptor binding energies could be estimated by photoluminescence to be 30 and 130 meV, respectively. By synchrotron-based spectroscopic ellipsometry the complex dielectric function up to an energy of 20 eV could be determined. Comparison with ab initio calculations allows an assignment of high-energy features to the peculiarities of the band structure.