The .45 GAP has about the same velocities for a given bullet weight as a .45 ACP. This does not include .45 ACP+p loads, just standard pressure.
The bullet diameters and the bullets themselves are the same as those used by .45 ACP cartridges.
Because the cartridge length is shorter, the .45 GAP has less space for additional powder. The increases in pressure could be bad.
So, given the same bullets and velocities, terminal effect would be the same. It then comes down to price and availability. Not everybody makes guns in .45 GAP so you would be complicating your ammo pile while losing versatility. I just don't see it as worthwhile.
The only benefit I see to GAP is if you have small, childlike, completely un-manly hands and can't grip a real gun.
