With funding from the Getty Grant Program - part of the J Paul Getty Trust - conservators were able to X-ray the 9ft tall painting (right). It solved one of the perplexing art world mysteries: stylistically, the painting dates from around 1778, before the birth of the sitter's children. X-rays revealed Gainsborough added the figures of the two sons later.
The work is among a selection of English portraits from Van Dyck to Lawrence on loan from the troubled gallery to the Olympia Fine Art and Antiques Fair which runs until 23 November.
The gallery said last January it could not continue on its present funding basis and it wants the public to adopt three of its Old Master portraits. It needs pounds 2,500 for Van Dyck's Lord George Digby, pounds 2,000 for William Dobson's Richard Lovelace and pounds 1,500 for Mary Beale's A Young Man.