We report a high efficiency sub-6GHz wideband Envelope-Tracking Power Amplifier (ET-PA) including a softswitching buck-converter and a wideband RF-PA, used in conjunction with a digital front end (DFE) environment for signal generation and predistortion. Both buck-converter and RF-PA were fabricated using an 0.15um GaN HEMT process. The DFE can generate the input signals for both buck-converter and RFPA, capture the output signal for feed-back and perform digital predistortion (DPD). The overall ET-PA achieves total efficiency of 39.5-46.7% and output power of 31.2-31.5dBm over 3.6- 4.0GHz with 20MHz LTE signals. With DPD, ACLR was below -45dBc with total efficiency reaching 47% at 3.6GHz. This, to the best of the authors' knowledge, is the highest efficiency yet reported for an ET-PA at this carrier frequency.