Originally, this was an Elizabethan-era word meaning "a boy who has sexual relations with a man." Nowadays, it is rarely-used slang for a gunman. You may be thinking that's a mighty wide gap between original meaning and modern usage, and you'd be right.
The newer meaning came about because mystery writer Dashiell Hammett, while writing his hard-boiled crime classic "The Maltese Falcon," was trying to refer to Wilmer, the cheap thug employed by Kaspar Gutman, as a homosexual. He figured his publisher would balk at actually using the word "homosexual" or any associated slang terms, so he used "gunsel" instead, assuming that most people would not know the true meaning of the word.
As it turned out, "gunsel" was quickly adopted, both by the public and by the underworld, as one of the slang terms for "gun-toting gangster." It's a good thing those gun-toting gangsters didn't know what Hammett really meant...