I believe that the GAP is for glock .45's and Auto and ACP I believe are the same thing. Someone will surely correct me if i am wrong. I do not know what the differences in GAP and ACP is.
ACP = Automatic Colt PistolThe .45 GAP is the same diameter as the .45 ACP pistol cartridge, but is shorter than the .45 ACP â€" and even slightly shorter than the overall length of the 9x19mm Parabellum and .40 S&W â€" and uses a small-pistol primer as found in smaller calibers such as the 9 mm Para, instead of the large-pistol primer as found in most commercial loadings of the .45 ACP (a few manufacturers occasionally use small-pistol primers on some of their .45 ACP offerings).
.45 AUTO = synonymous with .45 ACPThe .45 ACP (11.43x23mm Automatic Colt Pistol) is a rimless pistol cartridge designed by firearms designer John Browning in 1904, for use in his prototype Colt semi-automatic .45 pistolâ€"a design which evolved into the M1911 pistol adopted by the U.S. Army in 1911.
The .45 ACP would become one of the most successful cartridges of all time, among both military and civilian users. It has been used in innumerable handguns and submachine guns since its introduction.