I think that it is the middle ground cartridge between the 9mm and the .45 ACP. I would consider the .40 over a 9mm if you think that the 9mm does not have the power you want. With the .40 expect a snappier recoil compared to a 9mm. When considering the price of ammo, 9mm is going to be cheaper than .40 or .45 ACP.
I think that the .45 GAP is probably still going to be overshadowed by the .45 ACP. Its basically the same cartridge with a small pistol primer instead of a large pistol primer. I would rather have a .45 ACP than a .45 GAP for the simple fact that it has more than a century of proven effectiveness.
For all intent and purposes, I think that if it comes between 9mm or .40 I would rather have a 9mm. It is cheaper to shoot, less recoil, and with appropriate bullets, plenty of oomph to handle a two legged threat.