GaP has been shown to have good photo-catalytic activity in pyridinium catalyzed CO2 reduction. The photo-excited electrons in the conduction band of GaP should have a sufficient reduction potential to drive the reduction of pyridinium and CO2. In this work, we have studied water adsorption on the GaP surface using density functional theory calculations, and its effect on the band alignment. Our calculations have shown that there are surface states present near the band edges due to unsaturated dangling bonds, and water adsorption can remove these states partially or almost completely depending on the adsorption states of water. More importantly, we have found that water adsorption has considerable effects on the band alignment, shifting up the band positions by up to 0.5 eV compared to the bare surface. The computed level of the conduction band with the adsorption of water is rather close to the reduction level of pyridinium ions, thus suggesting that photo-excited electrons are thermodynamically possible to reduce pyridinium to pyridinyl radicals that further help CO2 reduction.