The GD and ED$_{n\geq2}$ are not respectively nested because one cannot reduce either model to be equal to the other by choosing particular values for their parameters. Now, let us cut to the chase. The gamma distribution has a shape parameter, and SET formulas do not. As a consequence, SET formulas do not fit the derivative of blood plasma concentration, because they lack shape parameters, and gamma distributions, or their convolutions, do in fact fit the derivatives. In the case of drug persistence in the body, without proper derivative fitting, there is no hope of predicting future plasma concentration of drugs using SET heuristics, whereas fitting of derivatives may permit more exact extrapolation. When one plots SET derivatives from actual data fits, the result is a wiggly curve, with one bump for each exponential term, which is pathognomonic for overfitting. For another example of a non-nested models exhibiting problematic AIC values, in that case for $n$ of small to moderate size, see this answer.