Gangs of New York found new fame as a movie by Martin Scorsese, but it has its origins in a 1928 potboiler of a history book, Herbert Asbury's Gangs of New York: An Informal History of the Underworld. The book covers gangster and criminal history in nineteenth century New York. The author researched police records and other documents, but he relied heavily on oral and popular accounts, and he writes with an eye for sensationalist entertainment. The gangster "Mighty Mose" may have really existed; I doubt he was capable of uprooting an entire tree or a lamppost to use as a club, or swimming vast distances without breathing. This is the Mose we see in the book, and he suggests the spirit in which the book must be read.
The movie takes a distorted account, and adapts freely. Scorsese collapses elements from several periods of history into one storyline. Bill "the Butcher" Poole existed, but he was murdered in 1855, which is probably why Scorsese calls him Bill Cutting. A Nativist such as Bill also wouldn't have lasted five seconds in Five Points, which was populated by recent immigrants. Five Points itself was built on Collect Pond, filled in in after being heavily polluted by industry; it lacked the elaborate catacombs presented in the film.
I thought it was odd that an Anglo-Saxon Nativist would hang out in an Asian brothel. Pete Hammill of The New York Daily News has done the research on that one. In 1863, there were about 25 Chinese residents in New York and, at any given time, a crew or so of sailors from Asian vessels. The Gangs couldn't have frequented this establishment. Chinatown did not exist until the late 1800s, and neither did Scorsese's opium dream of a brothel.
Scorsese also takes creative licence with the level of violence he depicts. In 1846, the year of the gang warfare with which the film opens, New York recorded 10 homicides (Eric Monkonnen, Phd, quoted in "Gangs of New York: the Errors"); the movie's establishing sequence suggests a bloodbath. The movie ends with the Civil War-related New York Draft Riots of 1863. Too many people died in this event, but the violence, hideous though it was, did not amount to the battle depicted in the film-- and relatively little of it took place in Five Points.
Both Asbury and Scorsese create gripping entertainment, but their work must be viewed as literary and cinematic legend-building, rather than accurate depictions of history.
Bernard Bailyn et al. The Great Republic: A History of the American People. Second Edition. Lexington, Massachusetts: DC Heath, 1981.
"Gangs of New York: The Errors." http://www.vny.cuny.edu/gangs.html
Pete Hammill. "Trampling City's History." New York Daily News. December 14, 2002.
Robert Seigel, Tyler Anbinder. "Gangs of New York." All Things Considered. National Public Radio, December 24, 2002. http://hnn.us/comments/6212.html
Robert W. Snyder. "Gangs of New York Gets New York City Wrong." openDemocracy. http://www.opendemocracy.net/debates/article.jsp?id=1&debateId=67&articleId=890