The general theory of relativity can be written in tensor notation at G + gL = 8pi T. [See source] In which case, the LHS is the geometric curvature of space time (the origin of gravity effects) and the RHS is the mass-energy tensor that warps space and time. L is the cosmological constant, which Einstein later admitted was a big mistake. As each term is a tensor, each term represents a set of equations...up to ten each depending on some assumptions.
Unlike the special TOR, the GTOR does not assume the reference frames to be non accelerating. And that's why the GTOR is more "general" than the special TOR. In fact, the GTOR starts out with the little thought experiment that says in effect: if we ride in an elevator without reference to the outside, how can we tell if the force we feel on our feet is from the elevator accelerating upward or from the force of gravity? The answer is...we can't. They are equivalent; so they must come from the same base cause. And that cause, Einstein found, is warped space and time.
The GTOR has been validated over and over by observing so-called gravity lenses. These are points in deep space where nearby massive stars bend space and time into lens like shape. So light passing nearby is bent and focused just like it would be when passing through a glass lens. Gravity lenses are observed all over space, especially near massive stars like neutron stars.
I recommend you read "The Elegant Universe" by Brian Greene who spends several chapters on the GTOR and how it has be validated through observations.