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.45 GAP was invented by Glock to fit in their 9mm/.40/.357 SIG sized frames. .45 ACP is too long and needs to use a thicker frame which produces a thicker grip, and a lot of people don't like that.
It performs on par or just below standard .45 ACP. The reduced case capacity leads to higher pressure, so there's more perceived "snap" in the recoil, but not more recoil itself.
Also due to the lower case capacity, it can't be loaded up to the same levels as .45 ACP. .45 ACP +P and +P+ will outperform GAP every time.
A few years after Glock invented .45 GAP they came out with the "SF" frames for .45 ACP and 10mm pistols. This let them use .45 ACP and 10mm with frames (and grips) closer in size to the smalle 9mm/40/.357 SIG frames. This more or less made .45 GAP obsolete immediately.
All manufacturers apart from Glock have discontinued their .45 GAP firearms. They're only continuing to produce pistols because a few American PDs adopted it when it first came out. As soon as they switch to something else you can expect it to be discontinued completely.
Also since it never really caught on and one company makes guns for it, ammo is kinda hard to find and fairly expensive.
tl;dr it's a mediocre, less versatile yet more expensive copy of the .45 ACP made for people with smaller hands that became obsolete a few years after its inception.