The Shorter OED defines Gunsel as “1 A naive youth; a homosexual youth, … 2 An informer; a criminal, a gunman.” Merriam-Webter’s Unabridged is similar.
The older of the two meanings, in the U.S., was “homosexual,” and the word was used with only this meaning in Dashiell Hammett’s novel The Maltese Falcon. However, when that novel was translated to the screen in 1941 (starring Humphrey Bogart), the film was “one of many of the era that … could only hint at homosexuality.” [Wikipedia] The hint was subtle indeed. When one character who frequently brandished a gun was repeatedly called a “gunsel,” most moviegoers thought that the unfamiliar word meant “gunman” or “gunslinger.” This mistake, owing to the fame of the film, eventually became an accepted alternative meaning.
Pauline Kael once mentioned that she had a very high regard for the 1941 film, for its original cinematic qualities and pacing. However, the film is, almost shot for shot, a literal translation of the novel. Many of its virtues are not original in film, but were derived from the book. It’s possible that Kael hadn’t read the novel (at the time, I didn’t venture to ask her that, because I hadn’t read it then, either.)