Energy gap dependences of solvent dynamics effects on the electron transfer rate in polar solutions are investigated by solving the diffusion-reaction equation. New points in this study are that (1) we treat non-linear response systems where the translational motion couples non-linearly with the rotational motion and (2) we investigate systematically the energy gap dependence of the solvent dynamics effect for the charge separation (CS) and charge recombination (CR) reactions. The non-linear free energy functions are characterized, based on the Monte Carlo simulation data for model systems where solvent translational positions are either allowed to move or fixed. The calculated results are that so far as the non-linear coupling is substantial, the energy gap law of the CS reaction is little affected by the solvent dynamics, while that of the CR reaction is greatly modified.