The .45 GAP has the same diameter as the .45 ACP pistol cartridge, but is slightly shorter, and uses a small-pistol primer instead of the large-pistol primer most commonly used in .45 ACP ammunition. Originally, the maximum bullet weight of the .45 GAP was 200 grains (13 g). In order to provide terminal ballistics on par with the standard 230-grain (15 g) .45 ACP loads, the .45 GAP was designed to operate at a higher standard pressure—roughly equivalent to the higher pressures found in .45 ACP "+P" rounds. Since the .45 GAP has a much smaller cartridge volume than the .45 ACP, the desired pressure and resulting velocity needed to be achieved through powder selection alone. Later development concluded that the .45 GAP could in fact fire 230-grain (15 g) ammunition just as the .45 ACP.
[edit] Glock .45 GAP pistols
The full-size Glock 37 pistol was introduced by Glock to use the .45 GAP cartridge and was followed by the compact Glock 38 and the subcompact Glock 39. The width of all three pistols is listed by the manufacturer as 1.18", compared with 1.27" for the .45 ACP Glock 21 and Glock 30, indicating that Glock was able to not only shorten the front to back size of the grip, but also the grip width.
[edit] Law enforcement application
Five state law enforcement agencies have adopted the .45 G.A.P. as a replacement to their current issue 9×19mm Parabellum (New York) or .40 S&W service handguns (Georgia, Pennsylvania, South Carolina, and Florida). The New York State Police, Georgia State Patrol, Pennsylvania State Police, South Carolina Highway Patrol, and Florida Highway Patrol[2] have all adopted the Glock 37 and .45 G.A.P.
Initially, due to its acceptance by law enforcement and the popularity of subcompact handguns for concealed carry, some manufacturers decided to produce pistols chambered in .45 G.A.P., including some Para-Ordnance M1911s and the Springfield Armory XD. Para has since dropped the G.A.P. from production and Springfield Armory has not had pistols manufactured in the G.A.P. cartridge for the last few years.
With subcompact pistols in .45 ACP I see why it hasn't taken off, first of all it was designed for just the Glock, second why not just shoot .45 ACP in a small pistol ?