It is not “crazy” to be interested in an obscure cartridge, if one is aware of the difficulty of obtaining ammunition and components. Going to the trouble of modifying cases, to hand-load the ammo, is a perfectly valid hobby. The usual cautions against using personally-hand-loaded ammo, for defensive purposes, would apply, so, I would see a .45 GAP pistol as a hobbyist gun, unless I were to manage to find a supply of properly-stored new-old-stock defensive ammo for it.
For a while, I liked the idea of the .45 GAP, mostly because Glocks that chambered 10mm and .45 ACP were too big for my hands, in the pre-Short-Frame days. Somewhere along the way, however, I discovered that a Glock big-bore Short Frame is a superb fit, especially in my left hand, which meant that there was no longer a truly practical reason to add .45 GAP; I really do like my G30s Short Frame. I have yet to complete my hand-loading set-up, and still do not have a clean, dry, really peaceful place to hand-load, anyway so, have not renewed any personal interest in the GAP.