Using first-principles calculation for the electronic structures of nm-scale [0001] GaN freestanding films, it is found that the Ga-terminated surface (S Ga) has a positive electrostatic potential, while the N-terminatedsurface has a negative electrostatic potential (S N), so that the energy bands tilt upwards from S Ga to S N. Additionally, it is determined that an intrinsic self-regulated charge transfer across the film limits the electrostatic potential difference across the film, which renders the local conduction band energy minimum at S Ga approximately equal to the local valence band energy maximum at S N. This effect is found to occur in films thicker than ~4 nm. If the dangling-bond/surface states at both S Ga and S N are passivated by pseudo-hydrogen atoms, the tilt of energy bands is similar, though the cross-film potential is reduced due to the extra H5/4-Ga and N-H3/4 dipole layers.