The common perspective on this is that Gregor Clegane is still "alive", in some sense. Qyburn’s use of Gregor for his experiments (touching on magic and necromancy specifically), which appear to be quite gruesome and murderous given the individuals given over to him who are killed as part of this experiment, suggests that he has been able to keep Gregor’s body functioning. Certainly, the "champion" that Qyburn alludes to (IV: 479, 656), for whom an extremely large and heavy suit of armor has been commissioned, would seem to fit Ser Gregor.  If this turns out to be the case, it will remain to be seen whether Gregor—or whatever has been made of him—is still capable of independent thought and action.