This contribution presents the AM/AM and AM/PM characteristics of a 6-bit Phase and Amplitude Setting Circuit realized in Gallium Nitride technology and operating at the Ku band. A test bench, based on three vector receivers and an absolute power reference, has been purposely devised to capture the deviation with respect to the linear behavior (known by the S-parameters) for both the magnitude and the phase of the vector response. The complete 64-state constellation is reported up to a 37 dBm of input power level, at which the effects of the static AM/AM and AM/PM distortion become evident, with about 3 dB of gain compression and 2.7 deg of phase conversion. The key figure of merit of the proposed test bench is the capability of operating with very high driving power levels (potentially up to 41 dBm), with possible applications in phased arrays, AESAs, and other signal conditioning systems.