I'm really enjoying Gazlowe in priest. Still trying out all kinds things but so far he's been insane everytime I get to play him. But so far it's been either a close win or a close loss. Still a ton of fun though.
I'm doing the same as well and it's going better than I expected. I first tried out Gazlow in Shaman/Druid because I thought that with cards like Savage Roar and Bloodlust, they'd benefit from the extra minions/board filling potential, but as people mentioned before, it's a bit too slow for that.
With Priest, it seems to work so well since you can get games to slow down and gain card advantage. Once that happens, you're good to go. One of my best games was getting a Cobalt Guardian, giving it taunt, then was able to follow up with a 2 cost mech.
Still making tweaks, but it's been the best Gazlow stuff I've done so far. One thing I need to get better at is using the Wild Pyromancer better since that's pretty much the sole card that'll push aggro decks into the late game.