The game Call of Cthulhu was originally set in the 1920s. Cthulhu by Gaslight presents an alternate setting in Victorian England of the 1890s. The supplement includes
- details of life in Victorian England, with essays on social class, occupations, crime, the cost of living, the government, the monarchy, and belief in the occult
- a map of Victorian London
- blank Cthulhu by Gaslight character sheets.
- The Yorkshire Horror, an adventure featuring interaction with Sherlock Holmes
In order to accommodate players' characters that have already adventured in the 1920s, referees are presented with several ways of transporting those characters to the 1890s.
In 1989, Chaosium republished Cthulhu by Gaslight, adding an essay on world politics of the time, a guide to prominent stores in London, and a glossary of Cockney underworld slang, as well as additional maps of the Tower of London, the British Museum, Westminster Abbey and Sherlock Holmes's residence at 221B Baker Street.
In the August-September 1987 edition of Space Gamer/Fantasy Gamer (No. 79), Lisa Cohen gave the book a thumbs up, saying, "Finally, what I have been waiting for! A game with real mystery and horror all in one."
In the June 1990 edition of Dragon (Issue 158), Jim Bambra thought that this book "perfectly captures the flavor and mood of the times, taking CoC into a previous age." Bambra also lauded the included adventure, saying, "The Yorkshire Horrors is a good period piece for CoC. It’s lengthy and develops well as the clues pile up, and there are plenty of opportunities for players to exercise their role-playing skills in Victorian society." With only slight reservations about moving characters to a period that has no other adventures written for it, Bambra concluded with a strong recommendation, commenting, "Gaslight is another excellent CoC product... Gaslight comes with my highest recommendation. Anyone interested in adventuring in the Age of Victoria should snap up a copy."
- Bambra, Jim (June 1990). "Roleplaying Reviews". Dragon. Lake Geneva, Wisconsin: TSR (#158): 90–91.
- Cohen, Lisa (August–September 1987). "Shedding Light on Cthulhu?". Space Gamer/Fantasy Gamer. (79): 38.
- "The 1986 Origins Awards". The Game Manufacturers Association. Archived from the original on 2012-12-16.