He was an authority on the work of J. M. W. Turner, about which he wrote three books and edited a collection of the artist's letters.
He was educated at Rye Grammar School and then at The Queen's College, University of Oxford, from where he graduated with a third class degree in modern history.
[2] Gage's early career was teaching English in Florence while he studied Italian art.
He completed a PhD in 1967 at the Courtauld Institute of Art under Michael Kitson that was published in 1969 as Colour in Turner: Poetry and truth.
[2] His master-work was his book Colour and culture: Practice and meaning from antiquity to abstraction (1993), which won the Mitchell Prize for Art History[4] and was translated into five languages.