Gregory turned professional after playing in the 2017 Walker Cup. He played a few events towards the end of 2017 and finished tied for 8th place in the Challenge de España. He struggled with injury in 2018 and had little success. He qualified for the 2018 U.S. Open, scoring 67 and 69 in sectional qualifying at Walton Heath Golf Club, his first major championship as a professional. He scored 92 in the first round of the U.S. Open, including 10 bogeys, 3 double-bogeys and 2 triple-bogeys and, despite a second round 75, finished last of the 156 competitors. However the season ended successfully when he earned a 2019 European Tour card through Q School after finishing tied for 11th place in the 6-round final stage.