I looked at one a few days ago. As AK says the biggest difference is that the GAP is a little shorter than the ACP. Its been out for some time but in a macho Blunder on Glocks part, there was no ammo available for almost a year. There is now three companies making ammo for it. At first I could see no Nich that it could fill but on more thought, for those already dedicated Glock shooter who's hands were not big enough to handle the Big Glock 45 but still want the stopping power of the 45 auto, this is thier gun. Performance of the two are nearly idenical. I handled it and the Glock 40. Thier size is about idenical. It should be a good round now that ammo is available.