The .45 ACP is to the .45 GAP, as the 10mm is to .40 S&W.
The difference is the .40 S&W was adopted by the FBI. At the same time, the 10mm wasn't an established catridge. The .45 GAP has an uphill battle, since the .45 ACP is an accepted standard.
It's a valid concept, otherwise the .40 S&W wouldn't be so popular. Truth is, those looking for more power on a 9mm frame will mostly likely do what the police do and select the .40 S&W. Too bad, I'm more a 10mm guy, but the .45 GAP probably deserves to be more popular.