General relativity is indeed a theory of relativity, despite the seeming contradiction you've raised. The term "relativity" in the context of Einstein's theory refers to the principle that the laws of physics should be the same for all observers, regardless of their relative motion. It implies that the fundamental laws of physics are independent of a specific inertial reference frame.
In general relativity, Einstein extended the principle of relativity to include gravity. He postulated that gravity is not a force acting between objects but rather the curvature of spacetime caused by mass and energy. According to this theory, the motion of objects is influenced by the curvature of spacetime, resulting in what we perceive as the force of gravity.
The equivalence principle, a key principle in general relativity, states that there is no local experiment that can distinguish between the effects of gravity and the effects of acceleration. In other words, if you are in an enclosed spaceship and feel a force pushing you against the floor, you cannot determine whether you are experiencing that force due to gravity or due to the ship's acceleration. This principle indicates that gravity and acceleration are indistinguishable in a small region of spacetime.
While acceleration is absolute in the sense that it can be measured in an inertial frame, the equivalence principle implies that the effects of gravity are indistinguishable from the effects of acceleration. This is why gravity can be considered relative in the framework of general relativity, as it is intertwined with the geometry of spacetime rather than being an external force acting on objects.
It's worth noting that the term "relativity" in the theory's name refers to the broader concept of the laws of physics being invariant under certain transformations, rather than implying a direct connection to the relative or absolute nature of acceleration.