Throughout his career Gunn received a wealth of commissions from eminent field-marshals, politicians, academicians, judges. As well as these formal portraits Gunn painted his wife and children and conversation pieces.

His second wife, Pauline, was the inspiration and model for some of his best known works. Glamorous portraits in the 1930s and 40s, include Pauline in a Yellow Dress (Harris Museum & Art Gallery, Preston) and Pauline Waiting. The relatively indistinct figures in the background are a foil for the sharply defined focus with which the sitter is depicted, her face, hands, clothes and jewellery being recorded with scrupulous attention to detail. The setting is Claridge's Hotel in London, which is identifiable from the livery of the waiters and the musicians, one at the piano and one standing at the back, who wear the red waistcoats worn by the Hungarian Quartet who played at Claridge's. The title of the work and the setting creates a slight frisson that possibly it is some secret assignation that Pauline waits for.

Gunn was born in Glasgow in 1893 the son of a prosperous tailor. He went to Glasgow School of Art and then the Edinburgh College of Art before going to the Académie Julian in Paris where he studied under Jean-Paul Laurens. His training was interrupted by the First World War, when he joined the Artists' Rifles. After the war he settled in London and turned his attention to portraiture, exhibiting regularly at the Royal Academy from 1923 onwards. His portraits were very well received and admired for the precision of his technique and his ability to capture a likeness. He undertook several successful group portraits, two of which are in the National Portrait Gallery in London. Conversation Piece at the Royal Lodge, Windsor depicts George VI and his family in a new and informal way, and Conversation Piece portrays the three writers G.K. Chesterton, Hilaire Belloc, and Maurice Baring in an effective and engaging manner.

Gunn was elected an Associate of the Royal Academy in 1953 and a Royal Academician in 1961. He was knighted two years later in 1963.