This paper presents, for the first time, a microfluidic tuning solution on a multilayer organic substrate to tune the frequency response of a GaN-based power amplifier (PA) from 2.4 GHz to 5.8 GHz and vice versa. By changing the fluid inside the matching networks microchannels, the packaged PA large signal response can be tuned allowing it to work optimally at the two specified frequencies based on the fluid in use. In this work, the fluids used are acetone and air and are meant to work for 2.4 GHz and 5.8 GHz respectively. The measured large signal data for the PA shows that Pout = 35.02 dBm, PAE = 36.41 % for Pin = 21 dBm at 2.3 GHz (acetone configuration) and Pout = 36.88 dBm, PAE = 44.03 % for Pin = 25 dBm at 5.7 GHz (air configuration). These numbers improve significantly for higher input power levels.