The portrait appears to be a copy of a lost portrait. It looks quite like another in the Vatican Gallery. These seem to be identified as Niccolò Marcello on the basis of its resemblence, not actually very close, to a medal in which he is named on the inscription. This medal may in fact be the source for the image, or it may, as the later inscription implies, copy a portrait by Gentile himself. Its date is unknown.
The National Gallery, Trafalgar Square, London WC2N 5DN