The .45
Glock Automatic Pistol cartridge is basically a shortened version of the
.45 ACP cartridge.
It was created to allow a .45 caliber round to be fired from a mid sized Glock frame, similar to the size of the Glock 17. The new Glock pistol that fires this proprietary cartridge is the Glock 37.
The .45 GAP has the same ballistics of the .45 ACP but has less recoil and a lower decibel rating.
This new cartridge is mostly targeted, no pun intended, at the military and law enforcement agencies although there still are no verified adoptions by any military or police agency in existence today. For civilians, there really isn't much use for this cartridge, we already have more than enough choices as it is.
One reason the military might have requested for this new design is to have guns that no other foreign power has ammunition for, thus, captured weapons will be of no use to them.