.45GAP is intended to duplicate .45ACP ballistics in a cartridge with the same basic overall length as 9mm and .40S&W so it will fit in a standard-sized Glock frame as opposed to the large G20/G21 frame.
This largely explains why few civilian buyers have embraced it; most shooters don't want a standard-size Glock that fires subsonic .45-caliber 180-200gr bullets badly enough to adopt a cartridge that only works in a handful of pistols, when .45ACP does the same thing and works in hundreds of them. Another factor is the appearance of new striker-fired polymer-frame pistols with a slimmer and friendlier grip profile than the older "Glock Block"- both from other gunmakers (the M&P45) and from Glock themselves (the 21SF).
The good news is that, if you handload, the .45GAP uses many of the same bullets as .45ACP, it can be loaded with common pistol powders in readjusted .45ACP dies, and it takes less expensive small pistol primers, rather than large primers like standard .45ACP brass. Commercial ammo isn't widely available or dirt cheap, but it can often be mail-ordered for reasonable prices, particularly if the seller bought too much of it and didn't realize how limited the demand is.
The bad news is that it only works in a handful of pistols- 3 Glocks, a Sp'fld XD model that was seemingly produced for about 30 minutes, and (IIRC) a Para-Ordnance M1911 that was produced for about 10.
AFAIK like other Glocks, the G37 uses polygonal rifling, and therefore cannot be used with inexpensive cast lead bullets unless you keep the barrel scrupulously clean. You can basically forget about purchasing cheap range-pickup brass; like a 10mm handloader, you may find yourself always taking a buddy to the range so he/she can watch where your brass goes.
One other little-known disadvantage of the G37/G38/G39 is that they have a slightly wider slide than other standard-frame models, so they don't fit in some Glock holsters made of rigid materials such as Kydex.