This paper presents a method for synthesizing the package of power GaN transistors in order to achieve wideband matching at harmonics. The proposed method is applied to optimize the package of a 15W power GaN HEMT for high efficiency performance over 39% bandwidth (S-band). It is demonstrated that the internal pre-matching of package ensures that the impedance seen by the GaN die at the second-harmonic frequency remains confined inside a high-efficiency region over a wide bandwidth whatever loads presented to the output of the package. In fact, when the 2nd-harmonic frequency termination of the package is varied all over the entire Smith Chart, the packaged transistor demonstrates a limited variation of PAE (70±3)% at 3.2GHz. Thereby, this method allows the designer to synthesize the external output matching of the packaged transistor only at the fundamental frequencies without imposing constraints on the 2nd harmonic loads.