I think some of you are too quick to write the GAP off for the following reasons.
I don't believe the XD45ACP was put out by SA to compete with the GAP. If they just put the gun out, it had to have been in development for the past couple of years at least. If they knew they were going to put it out, why would they design, build, and market the GAP's of their own, including in a 1911 profile. That is like the same owner building two McDonald's across the street from each other.
The Glock GAP's still fill a niche of their own. That niche is those people that WANT A GLOCK IN 45CALIBER but feel the Glock 45ACP's are too big for their smaller hands. There are people and agencies that want this and nothing but this, and they wouldn't or can't rely on the XD to replace that. Let's face it, the XD is not and probably will never be as trusted or relied upon as much as Glock. Yes, Glock does seem to have taken the harder route of redesigning the cartridge instead of the gun, but I am sure it costs less and takes less time to develop a new round, than it does a gun. The other thing is, it would probably cost Glock too much money to completely retool for a vastly different gun design, and they may actually have tried to do so, but could not get the mechanics right. As it sits right now, all they did was wind up with a redesigned slide and barrel. The frames are exactly the same as their smaller brothers, so Glock has saved a ton of money by not building something that has an in between frame size.
As far as metal Glock magazines are concerned. Does anyone know of a reliable metal magazine for a Glock? The only thing they are good for is destroying the plastic magazine catch. So then you have to install a metal magazine catch now, then you have to reinforce the plastic around the magazine catch, and since you thinned the plastic to make your gun grip smaller, you don't have much to work with, and your gun starts cracking, and .......
Although, I can't cite sources, I have read that the 45GAP is actually progressing faster than the 40S&W and 357SIG cartridges did. There have been departments that have adopted the round, and I doubt they are going to re-equip with XD's anytime soon. As a matter of fact, is there a single police agency that even issues XD's?
Has ammo been kind of hard to come by? Sure it has, but that doesn't seem to be a problem for me anymore and I just stock up on 2 or 3 thousand at a time. I save my brass for reloading, so by the time you subtract the cost of brass which I won't have to buy again, my GAP ammo is way cheaper than 45ACP, and the price disparity isn't that high between the two in the first place.
One last thing I would like to point out. The XD45ACP is somewhat of a revolutionary design to be able to fit that many rounds in the size package that it does. What is not to say that SA took a chance in thinning that material down in order to accomplish that, and they are relying on the fact that most people shoot their gun twice a year so it most likely won't break on them. Once they have these thousand round a month guys putting these guns through their paces, we might just find the new gun is a lemon.