I have a .45 GAP XD and I love it. I haven't had a more accurate out of the box shooter than it.
As far as the GAP round going to the grave...Gun guys are notoriously bad with change. Everyone laughed at the Glock "plastic pistol" and now almost every major gun company makes polymer pistols. There are at least three companies making GAP chambered pistols. Glock, SA, and I can't remember the third. Sorry, brainfart.
Many rounds have come down the pike and have been laughed at. .357 Sig, 5.7mm, 10mm, .17, you name it. Someone, somewhere even laughed at .45 ACP when it was first invented. They are all still around contrary to everyone's predictions. I don't think the GAP is any different. If the 10mm is still around, the GAP isn't going anywhere.
People say it's hard to find. I have no problem finding it. If your local guy doesn't carry it, ask him, he'll probably order and stock it for you. People say it's expensive, I have yet to see it priced more than .45 ACP. It's a good performing round. It's thicker case webbing will show it's worth when people start reloading them more.
While it was essentially a stepping stone caliber to get these polymer pistols to .45 ACP, it's not going anywhere soon.