From jb, right on cue:

There is NO longitudinal component to
Gravitational Waves.

...An ad hoc ancillary tacked onto GR to keep it in conformity with the
'no medium' premise. Same with the 'speed of gravity' being c.

A fact, is it???

Yeah. And if it ain't, and space is a
"void", then we're stuck with "fossil fields", 'virtual gravitons',
virtual photons, angels, imps, and Sky Pixies. oc