The crystalline quality, photoluminescence (PL), and photoconductivity (PC) of gallium nitride (GaN) nanowires (NWs) grown by chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) have been investigated and compared. The common single-crystalline structures and distinct long-axis orientation are confirmed for the CVD (m-axial) and MBE (c-axial)-GaN NWs. The PL examination reveals the difference of crystalline quality of the GaN NWs, which is difficult to differentiate by structural characterization methods. The MBE-grown NWs show sharper band-edge emission and much less defect emission in comparison to the CVD-grown NWs. However, it is interesting that the CVD-GaN NWs exhibit much higher photocurrent generation efficiency (or normalized gain) than those grown by MBE. The probable physical origins, including carrier concentration, surface state density, and surface polarity, that induce different surface band bending for PC, have also been proposed and discussed. The environment-dependent photoconductivities under inter-bandgap and sub-bandgap excitations were also observed. The systematic study concludes that the PL and PC behavior of this nitride nanostructure are controlled by two different mechanisms in the bulk and surface, respectively.