The applicability of the two-mode nonlinear model of decay stochasticity for explanation of the transition from monochromatic self-oscillation to developed stochasticity in the Gunn diode oscillator is demonstrated. Numerical realizations of the basic regimes corresponding to various cases of consideration of the weak nonlinearity of the falling portion of the current-voltage characteristic are presented. A comparative analysis of calculation results of time realizations and experimentally observed oscillograms of stochastic regimes is performed.