I know STR is based on two postulates viz. 1.All laws of physics are equally valid in all inertial frames of reference i.e There is no such experiment one can perform to know whether he is moving or not if he is in an inertial frame of reference. 2.The speed of light is constant(3*10^8 m/s approx.) in vacuum. But what about the postulates of GTR???Is it only the equivalence principle or something else?