Welcome to the forum Jackson. Enjoy!
I can't see the comparison. Size does matter and smaller is better in this case.I won't say for 100% sure but I could have swore that on the StarLine site is said that that the 45GAP used small pistol primers. That's probably true since they had to go with small pistol primers on the 40S&W so that the primer wouldn't hit the ejector on the 9mm size guns it was to be used in.
The GAP sounds interesting but I've got over 1100 rounds of 45acp on hand so personally, if it were me I'd just stick with the ACP since you can find it just about everywhere and it has a proven track record and the guns really aren't all that much larger then the GAP guns IMHO.