Batman: Gotham by Gaslight, originally published in 1989, is considered to be DC Comics' first Elseworlds story. According to the DC editorial copy, "in ELSEWORLDS, heroes are taken from their usual settings and put into strange times and places--some that have existed, and others that can't, couldn't, or shouldn't exist."
The Ferris Wheel was named after its inventor, George Washington Gale Ferris Jr. The reference to the attraction as a "Fox wheel" implies that in this world it was built by Batman's ally Lucius Fox.
In the orphans' first robbery, Tim is afraid and mentions what happened to Johnny Gobbs. This is the same line used in Batman (1989) when one of the muggers at the beginning of the film is afraid while sharing the loot on the rooftop.
When first discussing the Ripper's murders to the city council at the fair, Sister Agatha mention the Ripper "sent a kidney to the Gotham Gazette" and asks if that is an odd things to do. In real life, the actual Jack the Ripper did send part of a kidney of one of his victims to the police, in a parcel that also contained the famous letter "from Hell" and the signature that gave him his name.
Other than the Sherlock Holmes quote there are other Sherlock Holmes references such as The Diogenes Club [the club in Gotham by Gaslight was the Dionysus Club], Batman working with homeless children and Bruce Wayne even uses the Dancing Man code from "Sherlock Holmes and The Adventures of the Dancing Men" short story.