I can't imagine that .45GAP could really be considered unforgiving. It's theoretically less forgiving that .45ACP, but that's only because .45ACP is one of the easiest pistol cartridges to handload
That's the crux of it right there: It's more expensive and harder to work with than the cartridge it was designed to compete with, and offered in a gun that is unfriendly to the handloader to begin with.
There are plenty of downsides, with the only upsides being Glock collectors, folks who want something "different" and that it fits in a 9mm size framed Glock...... and those markets are pretty small: who wants a big gripped Glock that offers no real terminal performance advantage over a 9mm Glock (Modern 9mm personal defense ammo giving similar penetration and wound channels to .45 ACP)?
The 153 Glock Collectors in the world already have one, and the "Wanna be Different guys will dump theirs as soon as somebody they know has one too.... I don't see a future for an oddball "mass market" handgun cartridge.