This seems to be a weird trendy caliber and I dont really know why it was made. Just guessing I would say it was designed for a SMG with a really high rate of fire.
Scratch that, just read this.
Their requirement was simple... it has to be .45 caliber, and the overall length could not exceed that of the 9 X 19mm or .40 S&W cartridge because Gaston Glock wanted it to work with a pistol the size of the company's Models 17 or 22.
It is the same as a .45acp, even very similar performance, but is shorter so the guns chambered in it can have grips similar to a .40 or 9mm instead of the longer grips of a .45acp. It is good if you have small hands and still want a .45 but I don't think it will be a big success.
I think if they had made a single stack .45 ACP along the frame size of the 17, they would sell by the boatloads. The 36 is proof that it can be done, and I guess that's why Springfield went ahead and did it. Yes, I know their's is not a single stack, but it's still smaller than the 21 (which is so big because it started life as a 10mm!).
The isn't enough difference between the size of the ACP and the GAP to amount to a hill of beans. The only logical answer (other than Glock just wanted to have their name on a cartridge) is the one proposed by Pistolero about the countries that forbid military rounds for their citizens. I wonder how many countries actually have this restriction and what arm sells are like there anyway?
I have shot the G36 it is a very fine shooting and acurate gun. I think the .45 GAP will stay around but it fills a very small niche in law enforcement and i think that is were it will stay. Some police departments love the .45 but find the .45 ACP grip to be to large to fit all officers. So the .45 GAP fills the gap!