Would it be possible to transmit information through gravitational waves? First thing I've been wondering is how the gravitational field is emitted. Matter emits gravitational waves, and I guess that those waves travel at around the speed of light. If that's not the case, please direct me to something that explains that.
For now I'll assume that this emits a waves that travels at the speed of light.
Now those waves are emitted constantly without the apparent need of energy. Just the mass being there is enough to emit those waves, and the mass itself isn't consumed by the emission of gravitational waves. Again that's my assumption, please tell me if I'm wrong.
Now if we would be able to modulate the mass of an object, we would be able to modulate the gravitational waves this object emits. Hence we would be able to transmit information through those waves, if of course we'd have a device that detects gravitational waves with enough sensitivity.
Those waves would be transmitted at no cost, because the waves are emitted by the mass. The only cost of this transmission would be the "mass modulator", which has yet to be invented and which would require energy. However the actual transmission doesn't require energy, and the gravitational waves are harmless, unlike the EM ones.
So here comes my question (as in the title): would it be possible somehow to use the gravitational waves to transmit information?
 A: The JASONs put together a reasonably extensive report on the use of high frequency gravitational waves for anything practical (including communication) in 2008.  In the abstract they state that

the following are infeasible in the foreseeable future: detection of the natural “relic” HFGW, which are reliably predicted to exist; or detection of artiﬁcial sources of HFGW. No foreign threat in HFGW is credible, including: Communication by means of HFGW; Object detection or imaging (by HFGW radar or tomography); Vehicle propulsion by HFGW; or any other practical use of HFGW.

Here HFGW stands for high frequency gravitational waves, which includes the frequencies necessary for useful communication.  
A: First it's important to note that gravitational waves do require energy to produce. A good example of this is a binary pulsar, where the emission of gravitational waves carries energy away so the two pulsars spiral in towards each other and will eventually merge.
Having said this, it is theoretically possible to modulate a gravitational wave and use it to transmit information. You don't need a mass modulator, you just need something with a changing quadropole moment - the simplest example of this is a spinning dumbbell, and indeed this is basically what the binary pulsar system is. If you can change the rotation frequency you can frequency modulate the gravitational wave.
However gravitational waves are exceedingly hard to generate in the sense that very little of the energy of your system is carried away as gravitational waves. It doesn't seem likely we'll ever use gravitational waves for transmitting information.
A: There is another possibility for generating and detecting gravitational waves at microwave frequencies Something of a longshot though, but worth checking

Thin superconducting films are predicted to be highly reflective
mirrors for gravitational waves at microwave frequencies. The quantum
mechanical non-localizability of the negatively charged Cooper pairs,
which is protected from the localizing effect of decoherence by an
energy gap, causes the pairs to undergo non-picturable, non-geodesic
motion in the presence of a gravitational wave. This non-geodesic
motion, which is accelerated motion through space, leads to the
existence of mass and charge supercurrents inside the superconducting
film. On the other hand, the decoherence-induced localizability of the
positively charged ions in the lattice causes them to undergo
picturable, geodesic motion as they are carried along with space in
the presence of the same gravitational wave. The resulting separation
of charges leads to a virtual plasma excitation within the film that
enormously enhances its interaction with the wave, relative to that of
a neutral superfluid or any normal matter. The existence of strong
mass supercurrents within a superconducting film in the presence of a
gravitational wave, dubbed the "Heisenberg-Coulomb effect," implies
the specular reflection of a gravitational microwave from a film whose
thickness is much less than the London penetration depth of the
material, in close analogy with the electromagnetic case. The argument
is developed by allowing classical gravitational fields, which obey
Maxwell-like equations, to interact with quantum matter, which is
described using the BCS and Ginzburg-Landau theories of
superconductivity, as well as a collisionless plasma model. Several
possible experimental tests of these ideas, including mesoscopic ones,
are presented alongside comments on the broader theoretical
implications of the central hypothesis.

I understand that there is ongoing experimental work
