We have investigated the growth of GaSe, a layered semiconductor, on single crystal Al2O3(0001) (sapphire), an ionic crystal. We have used reflection high energy electron diffraction, x-ray photoelectron spectroscopy, and transmission electron microscopy and diffraction to study the growth, interface reaction, and resultant film structure. When a clean, heated (T = 620 degrees C) sapphire substrate is exposed to Ga and Se from a compound GaSe source (Ga,Se and Set) and a separate uncracked Se source (Se-x, x similar to 6), a polycrystalline film is formed with stoichiometry close to that of cubic Ga2Se3. After annealing to 1100 degrees C, the film evaporates, leaving a reacted interface layer containing both Ga and Se. Subsequent deposition on this reacted surface under the same conditions leads to growth of 500-1000 Angstrom grains of layered GaSe, which have their c axis normal to the substrate surface but random orientation parallel to the substrate. A mechanism is proposed that describes the formation of the interface layer and its effect on the subsequent growth of GaSe.