A first-principles study of intrinsic stacking faults in GaN is reported. Our calculations are based on density functional theory in local density approximation. We have found that the electron wave functions belonging to the conduction and valence band edge states tend to be localized almost exclusively on different sides of the faulted layer. We suggest that the electrostatic field caused by the macroscopically polarized 2H-GaN parts below and above a thin 3C-like layer around the stacking fault is responsible for these possibly shallow localized states.