Gregor dropped Sandor and was grimacing in pain. He never managed to pull the knife out because he got tackled through the tower wall, and even if he had managed to that isn't an indicator that the wound wasn't fatal. We like to call Gregor a zombie after his treatment, but he's not a wight.
Gonna have to rewatch that scene since you all seem to be saying the Mountain pulled the dagger out.
He seemed to be in some pain, at least at the point where Sandor impales him in the stomach and he grunts in pain as he pulls the sword out.
I mean, pain or not if Fujita manages to break one of his arms or legs (and I'm betting he'll manage this before zombie-Clegane) he's darn sure going to be hampered.
Hmm. On a second viewing, it seems to me Gregor was more just stunned/paralyzed than hurt by the knife through the head. I don't think he was capable of feeling pain any more. Still, I'm gonna go with that being a wound that would have been fatal eventually, though probably after Gregor killed Sandor with his own dagger had Sandor not tackled him through the wall.