Einstein wrote that gravitation, inertia, and centrifugal effects all arise from the motion of matter relative to the local vacuum in which it is embedded. He called this the GR ether.

Quotes from Einstein's "On the Ether" 1924

"The fact that centrifugal effects arise in a (rotating) body, the material points of which do not change their distances from one another, shows that this ether is not to be supposed a phantasy of the Newtonian theory, but that there corresponds to the concept a certain reality in nature."

and on Mach:

"He sought to escape the hypothesis of the 'ether of mechanics' by explaining inertia in terms of the immediate interaction between the piece of matter under investigation and all other matter in the universe. This idea is logically possible, but, as a theory involving action-at-a-distance, it does not today merit serious consideration."

and on the relevance of the GR ether in the face of advances in quantum theory:

"But even if these possibilies should mature into genuine theories, we will not be able to do without the ether in theoretical physics, i.e. a continuum which is equipped with physical properties; for the general theory of relativity, whose basic points of view physicists will surely always maintain, excludes direct distant action. But every contiguous action theory presumes continuous fields, and therefore also the existence of an 'ether'."

There you have it, from the good doctor himself. The paper is Chapter One of Saunder and Brown's book "The Philosophy of Vacuum". I will not elaborate on my interpretation of this ether concept, although I have summarized it on ATM.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924