A highly efficient two-stage 15 GHz fully-integrated GaN power amplifier (PA) designed for 5G communication is reported. SPICE simulations show this two-stage PA achieves an output 1 dB compression POUT, 1dB = 32.2 dBm with 28.2 dB gain and 30.0% PAE (power-added efficiency) for CW operation at 15 GHz. Its PAE reaches 38.7% at POUT = 34.0 dBm with a gain of 22.0 dB. Simulations also suggest that dynamic supply modulation from 28 V to 10 V may modestly improve the PA's efficiency at power back-off. When the PA is driven with 5/10/20 MHz LTE 16QAM modulated signals, the simulated output spectra and adjacent channel leakage ratio (ACLR) at POUT, 1dB = 32.2 dBm passed the LTE spectrum emission mask (SEM) without any predistortion (Pin = 4dBm), and the wider modulated signal bandwidth from 5 MHz to 20 MHz does not noticeably worsen the PA linearity. State-of-the-art literature survey suggests this highly efficient and linear GaN PA may be quite attractive for 5G PA applications at 15 GHz.