This paper proposes a simultaneous output power- and gain-matching technique in a sub-THz power amplifier (PA) design based on a maximum achievable gain (G(max)) core. The optimum combination of three-passive-elements-based embedding networks for implementing the G(max)-core is chosen considering the small- and large-signal performances at the same time. By adopting the proposed technique, the simultaneous output power- and gain-matching can be achieved, maximizing the small-signal power gain and large-signal output power simultaneously. A 150 GHz single-ended two-stage PA without power combining circuit is implemented in a 65-nm CMOS process based on the proposed technique. The amplifier achieves a peak power gain of 17.5 dB, peak power added efficiency (PAE) of 13.3 and 16.1 %, saturated output power (P-sat) of 10.3 and 9.4 dBm, and DC power consumption of 86.3 and 52.4 mW, respectively, under the bias voltage of 1.2 and 1 V, which corresponds to the highest PAE, gain per stage and P-out per single transistor among other reported CMOS D-band PAs.