(Sigh) The 45GAP puts 45ACP ballistics in a 9mm grip frame. That is its reason for being, and it does that. It is true that the 45ACP+P has better ballistics, but that is really beyond the intent of the 45GAP.
People gripe about the 45GAP mag capacity, but it is equal to or better than any 45ACP single stack. And as a double stack (more or less) tapered mag, it is easier to insert. 45ACP pistols with larger capacities tend to have bigger (usually much bigger) grip frames...and as the intent of the 45GAP is to keep the 9mm grip frame format, that is again outside the intent of the 45GAP.
The 45GAP is not popular among gun nuts, because most of us already have 45ACP's (who needs another caliber) and, if that includes big-ass double stacks, we have come to terms with that. If you are an LE administrator looking to issue hundreds of pistols or more, your frame of reference can be very different. 45ACP punch and capacity in a grip frame that already dominates, what, 70% of the LE market? And we are back to the 45GAP's reason for being. Further, not only will Glock give you that 9mm size grip frame with 45ACP punch, they can offer in in service, compact and supcompact sizes.
So it ends up being kind of a LE niche cartridge. Not that different from 357 Sig really.