"You know how you guys' kids grow up slower than human ones since being longer-lived, you have more time for that stuff? It's like that. But with bigger numbers. Lots bigger." So Gabriel's emotional maturity can be considered somewhere in the range of a human teenager's.

"Angels have an entire laundry list of powers and archangels have even more. They help for getting a look at what a person is really like. Sure, humans sometimes surprise me, albeit not always in good ways, but..." He shrugs. "Living on Earth among the mortals for a couple thousand years has given me a pretty good look at what they're like. And has resulted in me being more tolerant of things my siblings would frown on."

So here's a belated explanation for the way he talks. Humans have had time to rub off on him in various ways, including things like slang.