GPR data simulations require above all an efficient forward modeling algorithm. A Finite Difference Time Domain code is presented with Simplified Unsplit Perfect Matched Layer that leads to insignificant reflections on border with few modifications of the core algorithm. In addition, the Auxiliary Differential Equation method allows GPR simulations including dispersive phenomena that could not be neglected at radar frequencies. The numerical validation is done in a classic way for amplitude/time wave propagation and SUPML, and using the nonconventional Dispersion Analysis technique for physical dispersion behavior. Full 3-D and 2-D forward modeling results are finally presented for lossy, dispersive and random media in GPR conditions.