We studied the charge and potential distribution near the (100) GaP surfaces which bear the characteristic features common to polar surfaces of semiconductors. A self-consistent periodic quantum-chemical technique and a slab model are employed and the atomic and electronic structures for several surface stoichiometries are calculated. We compare these results with the predictions of the model approaches in order to check the limits of applicability of simplified models such as the electron-hole counting model or the model suggested by Nosker, Mark and Levine. We have found that zero dipole moment of the slab is an important criterion of the surface stability. The charges of the surface ions are flexible in our model and decrease strongly at the surface with respect to the bulk values. Therefore this criterion does not impose such severe restrictions on the surface stoichiometry, composition, and structure as are suggested in other models. We conclude that structure and stability of polar surfaces are determined by a balance of stoichiometric factors and electron density redistribution which should both be taken into account in a self-consistent manner.