Gregor Clegane is very dead. A shell now. The venom Oberyn used was too make sure that dude died for sure and in the most painful way just in case he won.
Mountain's reanimation and the wights btw are pretty close to the OG zombie lore. Haitian voodoo priests would raise the dead to do their bidding. Those zombies had no free will or sense of self and they weren't out eating folks brains.
In the books, Gregor Clegane's head is sent to Dorne as proof he's dead since Qyburn didn't need it. Bran Stark has a prophetic dream that shows him inside the Mountain's helmet is just made up thick and dark blood.
That dude gone imo.