Gregori / Nephilim / former Angel makes sense for several reasons. He's called "Watcher," specifically, and there are few "watchers" who are also 'out', in the supernatural/religious sense. Jim Butcher has stated that he's already told us what Mac is, but that you have to be a bit nerdy to really know, and so I'm assuming religious nerd counts. Being one of those myself, I think that this is pretty on the ball.
Furthermore, it's worth mentioning, in a meta sense, that Jim's described in detail his method of characterization, and especially how he describes characters. He chooses a bunch of different words and phrases that not only describe the character, but put in your mind a sense of who they are. Kind of like how Karrin is described as "someone's favorite aunt" a bunch of times, or how words describing Thomas routinely have a slightly more sensual tone.
Mac's descriptive language is often heavenly in nature - or, at least, how Mac's ale is often described. God is often referenced when Mac is described for the first time. He's always described as a man of unknown age, bald, wearing "spotless" white. He shows deference to Mouse, who is "little brother" to Uriel, and in all ways acts honestly, respectfully, and with integrity.
