It's only claim to fame is, it fits in 9mm/.40 size pistol frames & magazines.
And, until recently, with the introduction of the SIG P227, it was unique in that regard. This new SIG seems to have bridged the gap a bit -- but I say that without having actually held one.
Anyone who likes .45 and likes Glocks were forced to hold a 2"x4" (the Glock 21) or go to a lower capacity single stack .45 (Glock 36). I found the 21 a very "unnatural" gun for me, and wanted more capacity than the 36 (6 rounds).
I've had a bunch of different .45s over the years, and now have a Glock 38 and a SIG P220 Super Match. I find the Glock 38 easier to shoot than the SIG, and for everything but the most slowly-taken shots, just as accurate. Firing rapidly, it does fine.
As for ammo: I can generally find it as easily as .45, and my last bulk purchase from Georgia Arms, it was the same price as .45 ACP. I found .45 Gap during the great ammo drought over the past couple of years easier than anything else. Still looking for .22 in quantity, locally, however.
Most .45 enthusiasts argue that the .45 GAP is a solution to a problem that doesn't exist. Me? I like the cartridge, and in the Glock frame (17/22 size, or 19/23 size), it seems to lessen felt recoil better than any .45 I've shot.
Is there a reason to buy it? Depends on your needs, your hand size, and your sensitivity to recoil. It won't do a thing thing that the .45 ACP pistol can't also do -- except, maybe, fit in some smaller hands.
It's the gun I keep in my small bedside "home defense" gun safe, in case things ever go "bump in the night."