Hi,
There is nothing known as a gravitational shadow and neither can it exist.
In terms of Newtonian Physics, gravity is a force, in terms of general relativity it is a result of the matter all throught the Universe. Gravity is no entity. It is only a result of curvature of space-time and the spreading of matter all throughout the Universe.
How could a force have a shadow? In terms of relativity, how could something with no colour, shape,size anything or even frequency have a shadow? I think you have misunderstood something somewhere.
gagsrcool