I bought my Glock 37 without having ever fired the .45GAP. I have .45ACPs, so I did have that frame of reference to compare it to. I bought it primarily after reading of Florida Highway Patrol's extensive testing/search process before selecting the G37 Gen4 as their new issue gun. It was enough to make me comfortable trying it.
When I took it to the range the first time, I knew instantly that I'd found my new favorite caliber. I think it recoils less than the .45ACP, despite having the same or better ballistics. That's probably because of the smaller grip size giving me more grip in the hand and more control. I don't find the ACP to be difficult at all, I just find the GAP to be better. I might even go so far as to say much better. It's certainly enough that I bought a G39 to go with my OD 37 and I'll eventually add a Gen4 37...though the $550+ is a bit ridiculous for Gen4s.