I would not be afraid of buying the .45 GAP.
4 state police agencies have adopted the 45 gap; New York, Pennsylvania, Georgia, and South Carolina, and more are considering it, including Florida.
As long as that list doesn't shrink, I think it's unlikely that factory ammo will not be available.
This is the basic reasoning to why they like the GAP over the .45ACP or the .40S&W:
It fits in a smaller frame than the .45 ACP, which fit the majority of officer's hands better.
They like the proven stopping power of the .45 ballistics (.45 colt/.45 ACP/.45 GAP).
It does not have over-penetration issues like the .40 S&W.
In extensive testing, which came down to the .40 S&W vs. the .45 GAP, more officers preferred the recoil pattern of the .45 GAP to the .40 S&W, saying it was more of a "push" than a "snap" like the .40 S&W.
The .45 GAP is basically a .45 ACP shortened to the length of a 9mm. A minor benefit is that 45 GAP ammo is marginally smaller and lighter vs. 45ACP.
I've got a model 39 and love it.