Created by Ernest Durham in 2002, an engineer with Speer at the time. He did so at the request of Glock to create a cartridge with equal power to the .45 ACP, but be stronger in the neck area to lower risk of blow outs there, and also let it fit in the 9x19 & .40 S&W sized handguns. Of course GAP stood for "Glock Auto Pistol". No sooner than this cartridge hit public knowledge we started hearing "it will be gone in 10 years". I laughed just last week as I was in a local gun store and heard the same thing. "the .45 GAP will be obsolete in 10 years" and I couldn't help but reply with "yeah, that's the same thing they said in 2003.
The .45 GAP operates at the same PSI as the .45 ACP +P pressures and pretty well matches the standard ACP's performance. The Glock 37 holds 10+1, the Glock 38 holds 8+1 (yes same as a 1911 in a smaller lighter package), and the Glock 39 holds 6+1. I personally carried a Glock 39 for several years and never felt under gunned with those 200 grain Gold Dots. Then there is extra comfort in having a Glock 37 mag as your backup. Maybe it was just me but I always found them to be very soft shooters considering they match the ACP +P pressures, and accuracy always came easy as well.
The .45 GAP has a lot going for it when you really break it down. Sure , I know you're probably thinking "why do we need another .45 Auto when John Moses gave us the most perfect invention since sliced bread". Now why I do agree, J.M. Browning was a genius far ahead of his time, I do believe he would have liked the .45 GAP and may have been right there with the gunny pushing it. Lets look at this from a perspective recently brought up in a Glock Talk conversation. The .45 GAP is to the .45 ACP what the .40 S&W is to the 10mm Auto. Who's to say if the .45 GAP had hit first, that the .40 S&W might be barely holding its nose above water? You get smaller frame guns that almost any fingers can grip, plus, the GAP isn't as down on power vs the ACP as the .40 is to the 10mm. Had that one butterfly in China had flapped its wings at the right right time, our 1911 grips would be smaller, Glock 37/38/39 would be selling like hot cakes, and us 10mm lovers wouldn't have that horrible .40 Short & Weak bothering us.
There are many LEA's carrying the .45 GAP, but I cant seem to find a master list. I think the Alaska St Troopers do, as do the city police in Mt Airy, NC, I think GA is on that list as well.
I do recommend you give one a try at least once. If you can carry a Glock 19/23/32, then you can carry a G38 just as easy. The slide of the GAP's is a little wider 1.18" and loaded a G38 weighs 31.95oz, try that with a 1911! Lets face it, as gun enthusiasts, we hate to see any gun disappear, or go into expensive collector only status. Go out, find someone who still rents them, or watch GunBroker for deals. I got my OD Green G39 for $380, and found it well worth it.