I taught you every trick you know, Lucifer says, just before killing him; and Gabriel lies on the floor, shadowy wings splayed out, still as death.
Thing is, Lucifer's wrong.
About all of it.
Angels -- like demons -- can be killed. Gods can't. They can only be temporarily inconvenienced. Gabriel may be an angel, but he also isn't. Not just because of his stint in witness protection, playing Trickster until the Winchesters smoked him out.
No; Gabriel was Loki before he was Gabriel, and he was Trickster long before he was named Loki.
He is a god, and now he's pissed.