Do you have a source for this? I thought they were a direct prediction of GR and this link says the same: https://www.ligo.caltech.edu/page/what-are-gw

You can read about it in Abraham Pais' scientific biography of Einstein, _Subtle is the Lord_.

I don't have a copy of that book, but the abstract of this paper from 1937, co-authored by Einstein, claims that gravitational waves are a prediction of GR: https://www.sciencedirect.com/science/article/abs/pii/S00160...