Yeah if anything, Priest is a Tweener leaning face. Also Priest's entire story is the Judgement Day. It's the crux of his story arc right now. The Judgement Day continually getting in on Priest's business despite his many objections. Gunther is just the next challenge for him, and is likely the culmination of his title reign. He has continually gotten help or been the advantageous purveyor of circumstance around him (read: Drew/Seth keep getting fucked over because Punk or Judgement Day). What likely happens is Finn Balor, JD McDonagh, and Dom accidentally fuck over Priest at SummerSlam and Priest quits the Judgement Day.
Gunther also already inserted himself into the storyline multiple times, by taunting and goading Seth and Priest. He's the wild card that nobody knows what how he'll proceed. That's the story in of itself. Give it time and we'll see how this plays out. Heck, we might even get Imperium vs. The Judgement Day.