I've reloaded a lot of 45gap for both the G37 and G38. I think the cartridge is fantastic. I get the same velocity out of bullets up to 230gr while using about 1 grain less powder than 45acp. I use everything from 180gr jacketed to 255gr Keith-style hardcast lead in the 45gap, and it is an excellent performing cartridge.
I have owned a G21sf, and while the grip size was not a problem, neither was it ideal. Just because you can wrap your hands around a brick doesn't mean it is the best feeling or most ergonomic option available. I have large hands, but just don't like the feel of the G20/21 grip frame.
For me, the Gen3 full-size grip frame is a perfect fit. If the grip is of the RTF2 variety, even better. If I can get 45acp bullet diameter, weight, and velocity out of a gun that fits me perfectly, why would I settle for anything less. That's why 45gap works for me. I also own an M&P 45c, and while I really like it I wish I could get a backstrap insert that would give me the same backstrap angle as a Glock. Even the largest insert fails to add to the grip at the bottom of the frame, and that's precisely where I want more meat. For me, that portion of the Glock frame is what makes it feel so secure and solid in my hand.
I understand those who dislike the 45gap. If you have a 45acp pistol that works perfectly for you, there is no need for a shorter cartridge. But I love the 45gap, and think it was a great idea. It's too bad that other companies don't take advantage of the cartridge. A S&W 45gap shield would be a nice, small gun. A 45gap snub-nose revolver with a frame sized for the cartridge would be a nice little gun, too.
380 and 9mm have a similar if not identical relationship. If the 380 could match the velocity and bullet weight range of the 9mm, but still work in smaller pistols, who would complain. So what's with all of the negativity aimed at the 45gap. Seems kinda silly.