Comment by tsimionescu

A GUT would be a theory in which a single fundamental force/interaction exists - one that unifies the electro-magnetic interaction with the weak interactions*, and the strong interaction, and perhaps with gravity. But there is no reason why the universe shouldn't have 3 or 4 different fundamental interactions.

In contrast, QM/QFT can't deal with particles curbing space-time, and GR can't explain the movement of elementary particles, so we know for sure those two theories can't be completely correct: there must exist something we're missing to explain how gravity works at the lowest detail level.

* these are already unified to some extent as the electro-weak interaction, which as I understand is believed to be a single kind of interaction at very high energy levels, as seen in the early universe, with 2 different aspects at the lower energies typical of our age. I may be wrong on some of the details here though.