I'm a LEO in Miami, have been carrying the G37 for three years now. My Department issues the G22C and we have the option of buying a .45 caliber Glock at our expense.
I've shot both the G21 and the new G21SF, neither one 'fits', 'handles', feels right. The GAP gives me .45 caliber performance in the standard frame. I don't have any complaints with capacity, 10 rounds in the mag is fine. The accuracy and performance is equal to the ACP. I'm also the senior Firearms Instructor for my agency. We did all the testing with the three calibers, the 40 S&W, the GAP, and the 45 ACP. Barriers, clothing, glass, gelatin, etc.
For duty we spec the 200 grain GDHP for the GAP. It clocks at 950 fps and the expansion is typical Gold Dot. The caliber works as advertised. The Glock 37 has a thicker slide than the 17/22. I feel that it gives better balance and handling. As for ammo availability, I'm in Miami, FL. and I haven't been in a gunshop that doesn't carry it. Besides, there's always Midway USA.