A modified Z-scan experimental setup based on a Yb-doped photonic crystal fiber amplifier was used to achieve polarization-resolved measurement of the nonlinear properties of bulk intrinsic GaP. The experiment results reveal that three-photon absorption dominated the nonlinear absorption processe at room temperature. The anisotropy and crystal orientation dependence of the nonlinear absorption and nonlinear refractive index was experimentally studied and the nonlinearities showed saturation at high pump intensities.