The article https://vixra.org/abs/1804.0399 of S.J. Crothers indicates there is no possibility of deducing the speed of propogation of gravity waves except via linearization of Einstein's vacuum equations, and arbitrary choice of coordinate system wherein a coordinate divergence is assumed to vanish.
Crothers' key criticism is that coordinate divergence is not a tensor quantity, and therefore not a well defined object of GR. This was similarly observed by A.S. Eddington in his text on Mathematics of GR, c.f. https://vixra.org/abs/1103.0051
The above author is controversial, but the above articles are very interesting.