Haha, now that I really think about it I'm not sure that the 'up-down' view I was think about would work correctly unless all the bodies were on the same 2D plane.
If you were only looking at one body though, you could select it click a button and everything but that body goes away and you can zoom in and out to see how far and how much it's gravity effects stuff. You might need a reference for scale after so far though. You could use a rainbow like effect and something like a dark purple would be the least effect and farthest out, while red was closest and had the most effect. Somebody else might have a better way to implement this than colors though.