A euphemism for pornography suggesting quality periodicals and films for the discerning gentleman form the Victorian/Edwardian period (1880-1920). Almost certainly deriving from the euphemism for semen or ejaculate.
I shall retire to my man cave with my latest delivery of high quality gentlemen's relish and have a damned good wank.
Noun: Spunk, jism, come baby gravy. There is actually a brand of anchovy paste in Britain called The Gentleman's Relish - Patum Peperium. Coincidentally, it's rather salty.