What consequences should Gary face?
So, in the end, it can be everything from nothing to a criminal charge and conviction with loosing his job somewhere in the middle.
Even if you take "die slow motherfucker" literally, it's not a threat. A wish that someone dies is not a threat. "I will kill you" is a threat.
People talk shit all the time, a lot of people in this post need to calm down and stop being so quick to be offended.
Should have said it? Probably not. Does that make him a danger to anyone? Not by itself it doesn't.
Or of course, “Won’t someone do something about this troublesome priest”