Please take this with a grain of salt, since it is only my opinion. .45GAP stands for .45 Glock Automatic Pistol. It's a round that was designed for the Glock to rival the dynamics of the .45ACP.
I'm not sure of the statistics on the round, it just never really caught on. It was kind of like reinventing the wheel. I mean, the .45ACP already existed, so what was the point?
I used to work at a gun shop and I never saw either a .45GAP pistol or even one box of ammo sold. So, while it may be a good round and the Springfield is definitely a quality pistol, it's an orphan round.
Obviously, a $220. Springfield XD is a screaming deal. If you want to go with it, stock up on as much ammunition as you can. It may become pretty scarce someday.
Just my 2c.