Just a question. I'm not a Glock guy (or 9mm for that matter) and pretty much most 9mm mags I see are double stacked, so I just assumed this was the case as well. With regards to single stack, if it's a 45GAP or 45ACP seems to me not much of a difference in the mag widths as both will be roughly/exactly the same diameter?
Glock makes both single-stack and double-stack. The G36 is a .45ACP that carries 6 rounds single-stacked. The G30 is another subcompact .45ACP that carries 10 rounds double-stacked. Each frame and mag configuration has pluses and minuses. You want the most rounds? or the smallest pistol? You know you have to make compromises and pick the characteristics that are most important to you individually. There's no perfect model for everyone.
This whole concept is simple: Glock designed the .45GAP to attract a niche consumer: those who desire a .45ACP for it's effectiveness but with the ability to better conceal carry than the current .45ACP pistols. Almost everyone who carries a Glock concealed says it's not the overall size of the pistol they would like to improve, but the thickness of the grip, both depth and breadth. Glock has always offered the double-stack mags for the majority of their models because you will be better off with the maximum number of rounds the gun can carry. Stopping power is the cartridge, but fire power is the number of cartridges. The trade-off obviously is double-stacked mags are bigger around than single-stacked.
With the increase in conceal carry permits recently, it goes to reason there is a wider variety of people Glock must cater to.. The best gun for CCW is the one you are willing to carry everyday. If you decide it's too much trouble or uncomfortable to carry, the gun will do you no good sitting at home.
For CCW, size matters. The 9mm you are willing to use for EDC (Every Day Carry) is better than a .45ACP you can't conceal during the Summer months or which prints on your clothing (a major crime in many CCW states). If you can carry a .45 with the dimensions of a 9mm, with the only real detriment being 6 rounds vice 10, that will probably appeal to many. Additionally, if the mags for the .45GAP are the same size as the 9mm, spare ammo is easier to carry than the honking big .45ACP 10 rounders.
Just my thoughts. Everyone has to decide. Just like cars, cell phones, and every other consumer product, you can bat around the specs until it becomes another "Mac vs. PC" feud. In the end, the utility will be in the eye of the owner.