So what is the the 'theory of gravity'? And, have we developed an insulator for gravity?
As I stated previously, it is Einstein's "General Theory of Relativity." If you wish to know what that is, I suggest you avail yourself of any of the literally thousands of links on the 'net available to you by inserting the terms "General Relativity" in a search engine of your choice. The links will be better informed and more coherent than anything I could possible write on the topic.
Alternatively, you should read what "Physicist" has already provided on this topic on this very thread, and direct further questions to him.
And, have we developed an insulator for gravity?
Not to my knowledge. But if gravitation is a consequence of a curvature of spacetime (as Einstein's theory indicates it is), I doubt we ever will. As always, I defer to "Physicist," et.al., in matters such as these. Perhaps he will elaborate on this.
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.