Pure phase spatial light modulator can be used in many kinds of precision optics, and the prerequisite of correctly using this kind of spatial light modulator is the proper Gamma correction. We complete Gamma correction for a new type of Holoeye Pluto spatial light modulator by simply using the double-hole interference method and Webcam. The results show that the first order diffractive efficiencies of 8-level and 16-level blazed grating reach 70% and 80%, respectively; by displaying vortex hologram, LG01 doughnut beam is generated very well, which is sensitive to the aberration and the deviation of the phase modulated by the spatial light modulator. The stability of the modulated phase is also measured, and the variation upper limit or the averaged standard deviation is 2.43°.