Joining Brentford at a young age, Gelson progressed through the club's youth, 'A' and reserve teams before making his professional debut at age 19 in a 2–0 Third Division defeat to Halifax Town on 22 August 1961. Despite still being a part-time player, Gelson quickly established himself in the first team as a wing half, making 28 appearances during the 1961–62 season, a campaign which saw Brentford relegated to the Fourth Division.
