Thick GaN films, grown by hydride vapor phase epitaxy (HVPE), were separated from their sapphire substrate with a laser-induced lift-off process. After cleaning and polishing, these films offer the most direct way to investigate and compare the influence of crystal polarity on the electronic properties of Ga-face and N-face surfaces, respectively. Different barrier heights for Pt Schottky diodes evaporated onto Ga- and N-face GaN are determined from the dependence of the effective barrier height versus ideality factor by I-V measurements to 1.15 eV and 0.80 eV, respectively. The charge neutrality condition at the surface is modified by the spontaneous polarization due to the polarization induced bound sheet charge. This effect has to be included in the electronegativity concept of metal induced gap states (MIGS) and can also be illustrated by different band bending of the conduction and valence band, inferred from the self-consistent solution of the Schrödinger-Poisson equation. Furthermore, temperature dependent I-V characteristics are compared to simulated behavior of Schottky diodes, exhibiting excellent agreement in forward direction, but showing deviations in the reverse current.