A qualified optician, SJMG retired from this profession in 1982 to become a full-time ornithologist.
He was the editor of Birding World magazine until it ceased production in January 2014.
He also co-runs Birdline, a telephone information service containing news of rare birds, with Richard Millington.
Gantlett's specialism as an ornithologist is the advancement of bird identification.
Among the areas of knowledge in which Gantlett has played a key role are the identification of orange-billed terns.