In this paper, two vector nonlinear characterization procedures are presented, aimed at improving available GaN HEMT models for an accurate reproduction of the device behavior operating as a current source and in switched-mode RF power amplifiers. In the case of the more traditional linear amplifying classes, a technique for simultaneously extracting the higher order derivatives of the Ids(Vgs), Igs(Vgs) and Cgs(Vgs) transistor nonlinearities, along a desired load line, is described. This procedure conveniently isolates and models their contributions to the device intermodulation distortion (IMD) behavior. In the case of highly efficient switched-mode PAs, employed under drain modulation condition, a modified procedure for isodynamically measuring the higher order derivatives of the Vdd-to-AM and Vdd-to-PM amplifier profiles is put into consideration, as a way to refine the triode region reproduction for ON-OFF operation.