I've never heard of a "gallant," just a "top gallant" (usually "t'gallant," sometimes "gans'l"). I've sailed on ships with split t'gallants, though. I did sail on one ship with a skys'l, never a moonraker; I suspect those are both terms for "a sail above the royal".
Just a quick WAG: at one point it was the topmost sail, and then someone was like “what if we put another sail on top of that one so we can go seven speed?”
The next sail on top became the Gallant, and so forth, apparently topping out the tech with a Moonraker.