This is a very good question. The gravitational field of the universe is another name for space-time itself, so that although you can measure the wavelength of light with respect to space-time and the gravitational field, you cannot measure the dilation of space-time and the 'redshift' of the gravitational field with respect to something else external to it. Relativity again. The strength of the cosmological gravitational field, that is its 'radius of curvature' can, however, be measured via the so-called scale factor and it is this quantity that increases with time. Gravitational disturbances such as gravity waves, will indeed be redshifted as they wiggle their way across space. When we finally detect the 'cosmological background radiation' due to gravitational radiation, we will be able to determine their redshift and epoch of origin, and this will say much about how smooth the early history of the universe was.