[posix_me_less]

Quantum theory is about linear reversible evolution based on some Hamiltonian operator. GR is non-linear non-Hamiltonian theory (a naive Hamiltonian is zero); it has strange one-way solutions such as irreversible collapse of stars into black holes.

These two theories have very different mathematics and different concepts of state. Nobody has been able to connect them in a way that would be a "success" - a unified theory that explains both GR and QT in a consistent way.