There is no need for one, as there are no magnetic charges.
And, have we developed an insulator for gravity?
Electrical insulators prevent electric charges from moving. In the case of gravity, the charge is mass/energy. So what is needed for a gravitational insulator is something to prevent massive objects from moving. A brick wall will suffice for an example.
LOL - I thought a physicist would know the difference between an insulator and a barrier.