Gap fan here
Someone might as well chime in on the virtues of the caliber.
This summer I bought a glock 39 because I wanted to check the 45 caliber box. I already carried glock 26 and S&W 649.
I looked at the glock 45 acp offerings in G36 and G30, but both were too big to fit in my pocket. I'm pretty much limited to pocket carry and I wasn't interested in a range queen, therefore those options were rejected.
Three IDPA matches, 600 rounds and 8 months of daily carry later and I am absolutely in love with this gun. It fits in a standard 9mm frame and carries well once you get used to the chunk. The heavy slide makes it a soft shooter, even my wife handles it well.
As far as the "fate" of the cartridge, there are threads going back to 2011 expounding on how it was unnecessary, obsolete, an answer to a made up problem, and will surely be gone in 5-10 years.
While I will concede that if you are limited to buying factory ammo that it is not as easy to find as traditional 9/40/45acp. However, I am not limited to factory ammo.
Reloading the cartridge is a breeze. In a few months I have easily accumulated a few thousand pieces of brass. Starline and Hornady make new brass at a going rate of around $.30, but used can be found for .15-.20 each. Used magazines typically run $20, cheaper than other glock mags.
The best part about shooting GAP is that picking up your brass during a match is easy, since it can be separated from the ACP that isn't mine. In addition, most of the other folks know that I shoot Gap and my missed brass usually finds it way back to me.
The GAP has been a great gun to me, and I plan on using it for a long time to come, including in a TDI Ohio course and maybe a Tom Givens class or two.