Generation of proper source/load-pull impedances for a selected active device is essential to design an RF power amplifier for optimum gain and power added efficiency. As they are obtained, these impedances may not be realizable network functions over the desired frequency band to yield the input and the output matching networks for the amplifier. Therefore, in this paper, first, we introduce a new method to test if a given impedance is realizable. Then, a novel "real frequency line-segment technique" based numerical procedure is introduced to assess the gain-bandwidth limitations of the given source and load impedances, which in turn results in the ultimate RF power-intake and power-delivery capacity of the amplifier. During the numerical performance assessments process, a robust tool called "virtual gain optimization" is presented. Finally, a new definition called "power performance product" is introduced to measure the quality of an active device. Examples are presented to test the realizability of the given source-/load-pull data and to assess the gain-bandwidth limitations of the given source/load-pull impedances for a 45 W-GaN power transistor, namely, "Wolfspeed CG2H40045" over 0.8-3.8 GHz bandwidth.