Gallium nitride nanowires were synthesized on silicon substrates by chemical vapor deposition using the reaction of gallium and gallium nitride mixture with ammonia. Iron nanoparticles were used as catalysts. The diameter of nanowires is uniform as 25 nm and the lengths are 20-40 mum. The nanowires have single crystalline wurtzite structure with a few stacking faults. A careful examination into x-ray diffraction and Raman scattering data revealed that the separations of the neighboring lattice planes along the growth direction are shorter than those of bulk gallium nitride. The nanowires would experience biaxial compressive stresses in the inward radial direction and the induced tensile uniaxial stresses in the growth direction. The shifts of the band gap due to the stresses have been estimated using the experimental data, showing that the reduction of the band gap due to the tensile stresses can occur more significantly than the increase due to the compressive stresses. The temperature-dependent photoluminescence (PL) of the nanowires exhibit a strong broad band in the energy range of 2.9-3.6 eV. The PL could originate from the recombination of bound excitons. The strong room-temperature PL would be in line with the existence of strains inside the nanowires. The peak appears at the lower energy than that of the epilayer, which is consistent with the decrease of the band gap predicted from the x-ray diffraction and Raman data. The various strengths of stress may result in the widely distributed PL energy position.