We have obtained pair recombination luminescence spectra of four GaP crystals with different gallium isotopic compositions. The frequency shifts measured vs. average isotopic mass allow us to determine dE 0 /dM=0.19(1)meV/amu, where E 0 is the indirect gap and M the average gallium isotopic mass. From this mass derivative, a contribution of the gallium vibrations to the zero-point mass renormalization of the gap of 27meV is estimated. Subtracting this contribution from the total zero point renormalization, as determined from the temperature dependence of the indirect gap, yields a contribution of 59meV of the phosphorus vibrations to the gap renormalization. These results are discussed in comparison with similar data for group IV, III-V, and II-VI semiconductors. A comparison with theoretical calculations is also presented.