What is the difference between .45 ACP & .45 GAP calibers?

I'm interested in a handgun that shoots the .45 caliber. I'm not exactly sure what .45 GAP is and all I know is that .45 ACP is auto. From the guns I looked at, the .45 ACP gives a couple to a few more rounds, but what exactly is the difference between .45 ACP & .45 GAP? Is one more preferable than the... show more I'm interested in a handgun that shoots the .45 caliber. I'm not exactly sure what .45 GAP is and all I know is that .45 ACP is auto. From the guns I looked at, the .45 ACP gives a couple to a few more rounds, but what exactly is the difference between .45 ACP & .45 GAP? Is one more preferable than the other for different uses? This .45 caliber gun will mainly be for self defense, but also for some fun at the shooting range.
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