Electronic structure and optical properties of GaN systems doped with transition metals (GaN:TM, TM=V, Cr, Mn, Fe, Co, Ni) are studied with first-principles calculations. Influences of transition metals on electronic structure and optical properties are discussed. It shows that doped materials are direct semiconductors with half-metal property except Fe-doped material. Transition metal impurity introduces defect levels in energy gap of GaN, which is contributed by 3d electron states of transition metals. For GaN:V, Cr, Mn, Co, absorption peaks appear near defect level in low energy region. These peaks can be attributed to transiton between 3d electrons and N-2p electrons.