I've had a G37 for about eight months now, little over three thousand rounds through it. I also own a G22. As far as reliability goes, 100%. The GAP round is ballistically similiar to ACP, although it runs at a bit higher pressure. Feeding reloads or factory rounds through my G37 hasn't been a problem, some factory loads are impressive. I've shot almost every available bullet through my G37 and all have feed, extracted and ejected without incident.
I love the same size frame as well, the heavier slide fits nicely and the G37 has good balance. That heavy slide seems to tame recoil quite a bit, I believe my G22 has more recoil than the GAP round, even with hot loads. Speer 200gr. Gold Dots ran across my chrony screens at 1020fps. Even hot loads like this are tamed by the larger slide and heavier weight.
Factory ammo is about $20 a box for the American Eagle and Speer Lawman that has been available in Canada. Gold Dots and other defensive ammo can be found at P&D in Edmonton, factory hardball is available in lots of places. I have yet to see 230gr. Hydra-shoks, even in the States.
Some of the first G37's had some mag issues, the followers had too much play and caused failure to feed and some other problems, mine G37 has the new style magazines, with an improved follower. My six magazines all feed properly and mags are not an issue anymore.
An interesting side note, I am finding that out of the three Glocks I own, I am always wanting to shoot the GAP. I find myself intentionally leaving it at home just so I can burn up some .40 or 9mm rounds. I love having the same size platform in all my Glock pistols, I think the GAP round is well thought out and will have its place in the market. I long considered an ACP for my .45, but went with the .45GAP rounds for lots of reasons. I find the G21 pistol too much, even though I am 6'3" and have large mitts, the GAP is a perfect for me and my needs.