I chose a G37, because I love Glock pistols. I liked the idea of the same size frame as my G17 and G22. I shoot them all equally well and have come to love shooting the GAP. I have large hands but found the G21 to uncomfortable for me, so when the GAP cartridge was first introduced I was skeptical. However, after holding a G37 and a XD I liked the Glock way better, but hey I'm biased, I'd admit. Still longing for a .45 pistol of some sort, I went out on a limb and bought my G37. I have nothing but good things to say about it, I love the cartridge. I've tested almost ten factory loads now, and all are running at, or slightly more than .45ACP velocities, including +P's. An extra few fps isn't going to make much of a difference, so my first .45 pistol became a GAP. I'm sold, what else can I tell you?
The 45GAP XD is the same size as the other models in their lineup, they already had beefed up slides to allow an easy rechambering. Glock went with a larger slide after testing half a million rounds found some slide wear (duh!), if you are an XD fan, by all means, try out the GAP.