"I'm certain that Castiel is giving them a dressing down as we speak." No, Castiel doesn't understand the collar either, but he knows that it gives Gadreel a sense of calm that little had before he started wearing it. So he generally ignores it until those that visit make an issue of it.

He keeps combing his fingers through Gadreel's hair, letting the rhythm help to settle him. "Would you like to just kneel for me for a while, or do you need something more right now?"