The round is slightly softer than .45 ACP, but it was designed to replace the need for a large frame gun (ie. G21). Ultimately the choice comes down to how well the models fit in your hand compared to the larger .45 ACP models. I personally would suggest .45 ACP since it is widely available. .45 GAP is not a popular caliber, and can be hard to find at times. If you reload for it, the major difference is that the .45 GAP takes a small pistol primer versus the large pistol primer in .45 ACP.
Even Glock is giving up on the .45GAP with the "SF" .45ACP pistols in their lineup now.
It was a brute-force solution to a problem that the "SF" concept addresses more elegantly - make the grip smaller to accomodate the .45ACP round with a profile closer to 9mm as opposed to making the round a few millimeters shorter and claiming that new powders make it equivalent to .45ACP.
Pretty much what everyone else has said, the round never real caught on. Was a attempt by Glock to meet a need that was better served by a modification to their 45 ACP size frames while getting a vanity catridge with their name on it. There are folks who like having the smaller frame size though, thing is the upper is still larger then the rest of the glock small frames. Only nice thing about them is that Glock gave them all a little more rounded profile on the slide edges and stuff ala the G26.
If you find the G17/G19/G26 comfortable or just on the edge of your comfortzone for grip feel and control then the 45 GAP guns may be better for you then the G21/G30 SF models. Just be prepared to turn into a 45 GAP brass whore and hope you reload.
I've fired all three, and owned them at one point or another in the last two years through various horse trading. I found them a little too snappy for my taste when compared to the 45 ACP 21 and 30. With that and the scaricity of ammo and now reloading components, I let them go in trades for other things.