I think the .45 GAP fills a theoretical niche. Theoretically, there is a large call for a small 9mm to .40 sized handgun in .45 caliber. Realistically this demand exists but isn't at least in my opinion filled by the .45 GAP.
For starters the .45 GAP needs to capture a large segment of the LEO market to be successful, much like the .40 S&W has and the .357 Sig is really begining to. The .45 GAP is behind, WAY behind, especially when compared to it's competition the .45 ACP which has nearly 90 years worth of experience, production, and development in it.
I just don't see it happening, the .45 GAP is an expensive cartridge, the higher pressures used in the gun will be harder on the gun, and the practicality of the guns will be offset by the high cost. If you want a weapon in a 9mm or .40 size you'll buy a 9mm or .40, the ammunition will be more plentiful, it will be easier to shoot, and it will be probably serve you just as well.
I believe several here have quoted FBI(or some other recognized source?) statistics that show that a hollow point 9mm round and a hollow point .45 ACP round really don't vary that much power and penetration wise. Making both equally effective rounds. Sure a .45 caliber bullet makes us feel more comfortable and some will say damn the costs and use it. Personally, I'll be ok sticking with my 9mm, .45 ACP, .38 Specials, and even .32 ACPs.