It’s a pistol cartridge designed by Ernest Durham, .45 GAP, that was supposed to equal the power of .45 ACP but shorter to fit in a more compact pistol. GAP originally stood for “Glock Automatic Pistol” but became known to mean “Get Another Pistol.”
I considered it might mean one of those backless hospital gowns but a .45 cal. GAP sounds more likely, “Fall into the GAP” went the jingle.