I was very sad to hear about Larry's passing. Larry introduced me to the games Incan Gold and Groo (the card game), and I played Cosmic Encounter for the first time with he and Tadashi Ehara at DunDraCon.
If it helps, the Gaslight/Victorian era actually overlaps what we usually think of as the setting for the American Western, give or take a decade or so. The American Western is probably far more familiar to American gamers (and perhaps gamers around the world) as a reference point for technology, clothing, culture, and such, and the Weird Western has always been a popular setting for stories of the unknown and supernatural, even before the "real life" Victorian era.
Gaslight Cthulhu generally assumes a more urban (and typically English) setting than the Weird Western does, but that need not be the case.
So, in addition to the iconic gaslight lamps, expect proper ladies and gentlemen to be dressed in the stuffy Victorian fashion, transportation by horse-and-buggy and by steam locomotive, long-distance communication by snail-mail delivered by rail, and later by telegraph (but probably not telephone or radio), comparatively light urbanization and industrialization, a growing interest in science and a general faith in the overall benefit and goodness of science and industry, household servants (and, in the earlier part of the era, slaves) for the upper-class and often the middle class, and an idealization of the self-made "renaissance man" (who is in control of his own fortune, has created his own wealth, and has taught himself a little bit about everything he has needed to build his own home, invent his own industry, govern his own business and household staff, entertain and educate his own family, manage his own finances, fight his own fights in court and with fists, etc.) It's an era that seems to have placed a lot of stock in almost brutal practicality and realism, and which tended to distrust and disdain fantasy and sentimentality.
Some other setting possibilities in the same era that I think could provide some interesting backdrops for Gaslight-era Call of Cthulhu:
The Victorian/Gaslight Era was also a sort of "kitchen sink" for every kind of weird or outright insane idea or -ism you can possible imagine - starting with That Guy Who Invented Corn Flakes as an example of the standard-issue crack-pottery of the age, extending into every possible extreme of spiritualism, anarchism, communism, capitalism, fascism, nihilism, racism, classism, sexism, and beyond, on into embracing every imaginable weird cult, pseudo-science, bizarre health fad (such as swallowing radium pills or selling cocaine cough drops to kids over the counter), and screwball conspiracy theory of the time (hollow earth, secret continents, hidden dynasties of lost nations, mesmerism, the destinies of man being controlled by secret immortal cabals of evil foreign vampires, you name it....) In short, it was fertile ground for all sorts of the kinds of screwball stuff that turns up in Lovecraftian literature, dialed to 11.
In literary terms, the Gaslight era could be thought of as more than just the era of Sherlock Holmes - it also includes such early science fiction and popular Gothic works and interesting art movements as: