I own a box of .45 GAP, and based on that alone I will NEVER own a Glock 37, the gun it was developed for.
Anyway, all of your observations are correct. It was developed at Glock's whim solely for the new 37. The general idea was to give PDs that wanted .45 ACP stopping power the opportunity to go big bore without the huge grip of the 21 or having to buy new duty rigs, since the 37 was meant to be the same exact size as the 17 and 22. As it turns out, Glock had issues with the 17-size slide, and that is no longer going to happen.
The .45 Glock Automatic Pistol (I wish they'd mixed up the order to AGP; it's more stylish IMO) is quite literally 9mm sized; it is 11.23x19mm. It's short length precludes use of 230 grain bullets, crippling the round IMO. It's 200 and 185 grain loads are comparble, and the advertised pressure increase is modest, a .45 GAP runs two or three thousand PSI higher than a +P .45 ACP. This may sound like a lot, but remember .45 ACP is a modest pressure round to begin with. I'll believe it the precise moment I see one come out of a pressure gun.
I predict complete and dismal failure of this cartridge; as badly as we need a new service handgun cartridge, if only for the sake of variety, thr .45 GAP fills no perceptible niche. Anyone happy with.45 GAP would be just as happy with .40; anyone who really wants a .45 will settle for nothing less than Automatic... Colt... Pistol. And the 37 being forced to go big slide kills that cop market. R.I.P.
Just for the record, those service pistol cartridges I said we desperately need? We actually already HAVE them... they're the 10mm Norma and the 9x23mm Winchester... but they're hanging on to life by a bare thread, a thread constantly attended to by a group of devout enthusiasts, all of whom have my best wishes and support, because we need these cartridges and we need them in the mainstream where we can get reasonably priced factory ammo loaded with premium bullets and well built, properly timed factory guns.