Comment by cyberax
> But gravity is the curvature of space caused by mass, it is not described as a form of mass.
Gravitational waves carry momentum and energy (and therefore mass), just like electromagnetic waves. Theoretically, you can extract that energy from gravitational waves, by using oscillating masses tuned to the wave's frequency.
> Gravitational waves carry momentum and energy (and therefore mass), just like electromagnetic waves.
No, EM waves do not have mass. They are massless. They carry momentum and energy, yes, but not mass.
In GR, the source of gravity is not "mass", it's stress-energy. EM waves carry stress-energy even though they are massless.
Gravitational waves do have some aspects that are analogous to EM waves, but there is a key difference: gravitational waves do not have any stress-energy. They are pure spacetime curvature in vacuum. So while there is a sense in which they carry momentum and energy, since properly constructed detectors can extract momentum and energy from them, they do not carry any stress-energy and the momentum and energy they carry cannot be localized the way momentum and energy in EM waves can.