Left to right: 9mm Luger - .45 GAP - .45 ACP
Here's an excellent technical (very technical) writeup regarding the differences:
http://www.shootingtimes.com/ammunition/new_45/
So, what does all of that mean in layman's terms?
The .45 GAP helps to bring 45 caliber power to a concealed carry size pistol frame.
My understanding is that .45 ACP rounds are the original 45 caliber rounds used in all .45 caliber pistols until the advent of the .45 GAP. The .45 GAP was an effort undertaken by Glock and ATK/Speer to develop a smaller .45 caliber round for the purpose of developing a sub-compact .45 caliber frame that is much the same size as a 9mm. .45 GAP rounds are slightly shorter than a 9mm Luger round and much shorter than a .45 ACP round. Even with this difference, the .45 GAP, with its high-tech propellent technology, is able to keep pace with the much larger .45 ACP with regard to muzzle velocity and pressure using either 185 or 200 grain projectiles.
The .45 GAP is not merely a "shortened .45 ACP casing". There are physical difference in the casings including the rim characteristics and lead-in angle. Also, the .45 GAP uses a Small Pistol Primer, whereas the .45 ACP uses a Large Pistol Primer.
