Gamma nonlinearity of a digital camera-projector pair leads to obvious phase measurement errors in phase measuring profilometry (PMP). Among the conventional gamma correction methods, most of them use plenty of fringe patterns to implement gamma correction, which is unsuitable for real-time measurement. Based on the Fourier spectrum analysis of an orthogonal sinusoid grating, a correction method for the evaluation of the gamma value is proposed. The evaluated gamma value is then pre-encoded into computer-generated phase-shifting fringe patterns to obtain well-sinusoidal fringe patterns, the errors of the PMP are reduced. Only one pattern is needed and the defocusing effect of the measuring system is considered in the correction. Experiments have verified the feasibility and validity of the proposed method.