One of the posters seen in Gotham throughout the game is for a film called "The Ghost in Gray" and features a man in a hat and goggles. An actor named "Simon Trent" is also listed on the poster. This is a reference to the Gray Ghost, a character from "Batman: The Animated Series" and the eponymous character of the episode "Beware the Gray Ghost." The Gray Ghost was an old TV superhero played in universe by Simon Trent, with his costume consisting of a fedora hat, goggles and trenchcoat. Bruce Wayne was a huge fan of the Gray Ghost series and character, with Ghost serving as inspiration for Bruce to become Batman. In said episode, Batman enlisted Trent's help (in costume as the Gray Ghost) to stop a criminal who was imitating a scheme used by one of the villains from an episode of the Gray Ghost series.

Additionally in the Batman Animated Series, Gray Ghost/Trent was voiced by Adam West, who played Batman in the 1960s TV series. Adam West's Batman costume is available in Arkham Knight as DLC.