Gerwyn joined Ystrad from Ferndale RFC, playing both centre and wing. The powerful wing was influential for a number of seasons for both the 1st and 2nd XV, but this was particularly significant in the clubs Division 1 years between 2012 and 2016 where he was part of the 2015 Swalec Plate winning team. Gerwyn later finished his career in the back row and he was part of the squad which won the Division 1 title and playoff in 2017 which gained promotion to the National Championship.