As measurable gravity appears to emanate from or is present basically in the vicinity of large masses, eg the earth, I would expect a 'gravity insulator' to block/lessen the effects of 'gravity' from what ever it surrounds, not counteract them, as a rocket does.
You seem to be confusing fields with currents. An electrical insulator doesn't block electric fields, it blocks electric currents. In the case of gravity, you want your "insulator" to block the field, not the current. (The fact that there is no such thing, owing to the nonexistence of gravitational dipoles, is another question.)