The underworld slang word "gunsel" is not a son of "gun."
"Firearms" words generally are an interesting set in the arsenal of English.
It includes extended-sense words like muzzle, which moved from "device put over an animal's mouth to stop it from biting," to the mouth itself ("projecting jaws and nose of the head of an animal"), which is basically a reversal of sense, to "open end of a firearm."
It includes surviving words from obsolete technologies, such as cartridge, which comes from Italian cartoccio "roll of paper." In old firearms the charge was in a roll of paper or linen; it still was that way in the American Civil War.
Among the most interesting is gun itself.
In the mid-1300s, it's gunne, "an engine of war that throws rocks, arrows or other missiles from a tube by the force of explosive powder or other substance." And it's apparently a shortening of the woman's name Gunilda.
Male soldiery's identification of women with powerful weapons is common historically, from Mons Meg to Davy Crockett's Ol' Betsy to the British Army's Brown Bess to World War I's Big Bertha. Gunilda, the woman's name, is from Old Norse Gunnhildr, a compound of gunnr and hildr, both meaning "war, battle" (and both used independently as women's names), which makes it a logical choice.
Of course, the word gun might be directly from Old Norse Gunnr. But there's a specific Middle English use of Gonnilde for "cannon," and there's an Anglo-Latin reference to a specific gun from a 1330 munitions inventory of Windsor Castle ("... una magna balista de cornu quae Domina Gunilda ..."). Also compare gonnilde gnoste "spark or flame used to fire a cannon" (early 14c.). The word was perhaps influenced by or confirmed by (or possibly from) Old French engon, a dialectal variant of engin "engine."
The meaning of gun grew with technology, from cannons to firearms as they developed; it was popularly applied to pistols and revolvers from 1744. In modern military use the word is restricted to cannons (which must be mounted), especially long ones used for high velocity and long trajectory. But in general use, gun remains a very broad word, as noted by philologist Carl Darling Buck in surveying the map of Indo-European languages:
[G]un covers firearms from the heaviest naval or siege guns (but in technical use excluding mortars and howitzers) to the soldier's rifle or the sportsman's shotgun, and in current U.S. use even the gangster's revolver. In the other European languages there is no such comprehensive word, but different terms for the small or hand gun of the soldier or sportsman (even these, sometimes differentiated) and the heavy naval guns or artillery pieces ....
Most explanations for son of a gun (1708) are more than a century after its appearance. Henley (1903) describes it as meaning originally "a soldier's bastard;" Smyth's "Sailor's Word-Book" (1867) describes it as "An epithet conveying contempt in a slight degree, and originally applied to boys born afloat, when women were permitted to accompany their husbands to sea ...."
An interesting non-son of gun is U.S. underworld slang gunsel, which people who remember it at all probably think of as meaning "young hoodlum." And they probably remember it from "The Maltese Falcon."
But it's actually a hobo slang word from the early years of the 20th century meaning "a catamite;" specifically "a young male kept as a sexual companion, especially by an older tramp." It comes from Yiddish genzel, from German Gänslein "gosling, young goose."
The "young hoodlum" sense seems to be entirely traceable to Dashiell Hammett, who sneaked it into "The Maltese Falcon" (1929) while warring with his editor over the book's racy language:
"Another thing," Spade repeated, glaring at the boy: "Keep that gunsel away from me while you're making up your mind. I'll kill him."
The context implies some connection with gun and a sense of "gunman," and evidently that is what the editor believed it to mean. The word was retained in the script of the 1941 movie made from the book, so evidently the Motion Picture Production Code censors didn't know it either.
Hugh Rawson ["Wicked Words," 1989] writes: "The relationship between Kasper Gutman (Sidney Greenstreet) and his young hit-man companion, Wilmer Cook (Elisha Cook, Jr.), is made fairly clear in the movie, but the overt mention of sexual perversion would have been deleted if the censors hadn't made the same mistaken assumption as Hammett's editor."