It has been said that the Holy Roman Empire was neither Holy, nor Roman, nor an Empire.
The same thing has been said about the modern Grand Unified Theories. They are neither grand, nor unified, nor even full theories.
GUTs, in modern theoretical physics parlance, don’t actually try to explain everything. They content themselves with unifying 3 out of the 4 Universal Forces – the Strong Nuclear Force, the Weak Nuclear Force, and Electromagnetism – together into one force ruled by a single law. All modern GUTs ignore the force of gravity, since gravity is so much weaker than the other three. It should be noted, by the way, that the Electromagnetic Force and the Weak Nuclear Force have already been successfully unified into a single “Electroweak” force in this way; GUTs are all attempts to bring the Strong Nuclear Force into the game.
The more grandiose attempt to unify all four forces together, and to resolve the contradictions between General Relativity and Quantum Mechanics, is sometimes called “The Search for the Theory of Everything.” (A theory that merely smooths out the differences between General Relativity and Quantum Mechanics, without trying to reconcile gravity with the other 3 forces, would be called a Theory of Quantum Gravity; the best candidate for a workable Quantum Gravity theory at last count was Superstring Theory.)