If you examine the .45 GAP within its context, and put aside A) .45 ACP chauvanism, and B) contempt for the fact that ammo and components are currently scarce to nonexistent... the .45 GAP is an excellent cartridge. It's actually one of my favorites. Three best Glocks ever made: G37, G19 and G42. All you need. Naturally, on a S&W forum it's not going to be popular amongst the fandom.
The .45 GAP puts .45 ACP ballistics in double stack 9mm frame. That's a noteworthy achievement. The Glock 37 is also very accurate. S&W wouldn't chamber the M&P for it, for marketing reasons. If you shoot a 1911, it's also a worthless cartridge. If you shoot a revolver, you don't even care!