Gallium nitride films were grown by reactive rf magnetron sputtering on sapphire substrates. Crystalline (1120) GaN films were obtained on (0112) sapphire at substratestemperatures between 640-680 OC. High N2 partial pressures are required to crystalize the GaN films. Nitrogen incorporation and crystal quality of GaN films are examined as a function of substrate temperature and nitrogen partial pressure. Band gaps of 3.4 eV, and photoluminescence peaks as narrow as 11 meV are reported for sputtered GaN films.