We have studied molecular beam epitaxy grown GaN films using electric force microscopy to detect sub-1 mu m regions of electric field gradient and surface potential variations associated with GaN extended defects. The large piezoelectric coefficients of GaN together with strain introduced by crystalline imperfections produce variation in piezoelectrically induced electric fields around these defects. The consequent spatial rearrangement of charges can be detected by electrostatic force microscopy, and can be additionally modified by externally applied strain and illumination. The electron force microscopy signal was found to be a function of the applied tip bias, showed reversal under externally applied strain, and was sensitive to above band gap illumination.