You can find .45 GAP for about the same price as .45 ACP, from some sources, and there are also conversion barrels out there to let you run .40 S&W in a .45 GAP Glock. (I've been getting my .45 GAP ammo from Georgia Arms.)
I've got a Glock 38 which stays in .45 GAP, and a Glock 37, in which I often run a .40 S&W conversion barrel ) As noted, .45 GAP Glocks can be found cheaply -- I picked up a my Glock 38, NIB/unfired, for $300, and a used 37 for $300. After finding a .40 conversion barrel for the 37, it was still less expensive than getting a comparable Glock 22. (I liked it so much I added a more-costly trigger system upgrade.)
I really like the Glock 37 and 38... and shoot them better than MOST of the .45 ACP semi-autos I've owned. (An exception: my Springfield XDm .45 Competition which is simply outstanding.)