Personally, I never really understood the appeal of .45 GAP. It's like Glock was trying to copy the formula of Smith & Wesson without any of the demand to back it up nor any legitimate benefit aside from having a shorter OAL. (Yeah, yeah, I know .40 S&W isn't actually a shortened 10mm Auto, but that's how folks view it and that's how S&W presented it to the FBI.)
I mean, sure, if you absolutely must have .45 ACP performance in a smaller package, perhaps because your hands are too small to get a good grip on a standard .45 ACP pistol, then obviously it's your Goldilocks cartridge, but outside of that, what does it offer?
Furthermore, I've never understood why .40 S&W, .45 ACP, and .357 SIG aren't more popular in jurisdictions in which Military cartridges are illegal for civilians to own.
Instead they're into oddball cartridges like 9x21 IMI and .38 Super. Perhaps most puzzling is the fact that said cartridges aren't typically loaded any hotter than 9mm Luger, despite the fact that they obviously can be, so you'd think that if you can't carry 9mm Luger and are essentially forced to use larger, more expensive ammunition, then it might as well be more powerful to compensate you for the cost a bit.