My friend and I encountered this bug earlier today. It took a handful of tries, but we managed to take another shot at Gurtok. We switched realms, backtracked to retrieve another vial of Gurtok's blood, then headed back to Gurtok's chamber. Now when we defeated the enemies guarding the vial of blood, we weren't actually able to pick up another vial, but somehow or other, when we entered Gurtok's chamber again and used the brazier, we triggered the fight.
Not sure exactly what this is all about, but until KI is able to resolve this issue, I hope that this helps you!
I had the same thing happen. I took the vial in, and my next quest was to beat him. Then I left the cave before beating him to do something else. When I returned he was asking for the blood. I did NOT have to go back and get the vial again. I just went to the brazier and hit X. It then allowed me to fight him.
Definitely seems like a bug or something, but you don't have to do much extra to get to fight him.