Experiments on photoionization process of Gd by the atomic vapor laser isotope separation (AVLIS) method were carried out. The theoretic principle of selective photoionization of odd Gd isotope based on the polarization selection rules was also presented. Experiments validate the isotope selectivity with J=2→2→1→0 multistep photoionization of Gd. The effect of the magnetic field and the laser polarization state on the isotope selectivity was studied by the experiment and numerical analysis. When the angle of axes between laser polarization and magnetic field is 90°, the selectivity may be lost easily by an external magnetic field. The selectivity decreases obviously as the magnetic field exceeds 2×10-4 T. On the other hand, the selectivity changes much more slowly when the angle of axes between laser polarization and magnetic field is 0°.