The GAP was slightly a shorter round with a thicker casing. Shorter to fit in smaller sub compact firearms. The stopping power is the same between the two rounds. The GAP's are loaded to the equivalent of the .45 acp +p. if you're needing a subcompact with the .45gap and don't mind paying more for the ammo or reloading, go for it. Anything else, I'd definitely stick with the .45 ACP.