Vardorvis, Duke Sucellus, Leviathan, The Whisperer, Vorkath.
It seems counterintuitive/weird/outright stupid to me, that you do the quest, you kill the boss, then if you try and do the boss again you WILL get your ass handed to you.
The bosses could drop loot at comparitive rates as CG vs Regular Gauntlet. But it feels awful to do the quest, enjoy the bosses, then not be able to fight them