A model involving two coupled anharmonic oscillators (electronic and ionic) is applied here for theoretical computation of the second order Difference-Frequency Generation (DFG) and Linear Electro-Optic (LEO) susceptibilities of GaP crystal. The crystal of GaP belongs to III-V group compounds owing to a cubic zinc-blende type structure. Linear absorption is discarded for the selected spectral region of 0.112 eV - 0.332 eV. So, the contribution of the imaginary part of the involved complex linear ionic susceptibility i (1)() to the resultant DFG and LE-O susceptibilities is neglected in the computation of DFG and LE-O coefficients. All of the four constants (nonlinear strength factors), appearing in the model, are determined with the help of experimental data of DFG susceptibility observed in the region of 0.112 eV - 0.326 eV. Applying such calculated nonlinear strength factors in the concerned model expressions, DFG and LE-O susceptibility coefficients are computed as a function of frequency to illustrate and hence to study the dispersion in the region of 0.112 eV - 0.332 eV. Keywords: Nonlinear Optics, Difference-Frequency Generation, LE-O Susceptibility Coefficient.