[5] Despite still being a part-time player,[4] 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.
[5] Gelson was a key player in Brentford's 1962–63 Fourth Division title-winning season and made 32 appearances,[5] but he remained part-time and did not sign a professional contract until 1964.
[9] A "hard man" as a player, in 1970 he ended the career of Colchester United's Bobby Howlett in a challenge which resulted in a broken leg for the Us defender.
[2][12] After his departure from Brentford, Gelson played on in non-League football, appearing for Hillingdon Borough (two spells), Hounslow and Walton & Hersham.
[15] He worked for a number of years in Doha, Qatar as a loadmaster for Khalifa bin Hamad Al Thani's multiple private aircraft.