At least 6 state police agencies carry the 45 GAP including here in NY and PA.
Georgia State Patrol also carries the load.
It was something of a non-starter. The idea, as has been stated, was to get a .45-caliber loading in a gun that was the same dimensions of a 9mm. The initial promise and marketing from Glock was very emphatic on that point.
Why? Because law enforcement could switch guns without having to buy new holsters and mag pouches. That's a significant investment otherwise. So, the promise was made that the new guns would fit the old duty gear.
Except that the new guns weren't cycling as well as expected, and Glock had to make them a bit larger, thus losing compatibility with old gear. Oopsie.
So, the initial market was really soft, and ammunition makers weren't exactly chomping on the bit. End result? Gaston got his name on a cartridge. A few early adopters are stuck paying a high premium for ammo, and gun selection is pretty much limited to one of the Glock models at this point.