I think it wouldn't be slowed, blocked or dissipated by matter, other than interference with other gravitational waves or time dilation from passing through curved space. I'm not sure what benefit it'd have over neutrinos, though, since those share those properties.
Imagine a 100x100m neutrino detector, and how difficult it would be to hit that surface with a neutrino over a distance more than a light-year. Or conversely how strong your neutrino source must be if you radiated in all directions.
I bet high frequency traders would be pretty keen on a device which sent speed of light signals straight through the earth rather than round the surface!
Neutrino communication is simpler, you just need to modulate a nuclear reactor or a particle accelerator, and simply receive it at an underground detector array in a mountain... Well, easier than modulating gravity...