So they plan to fight us by throwing giant rocks at us, hmm? Decent, so we'll need a substantial defense network to throw off potential attack. We're also going to need to reassure the colonists that we have it under control, can't let panic grip the hearts of our people. Make sure they understand that we can stop this, show them footage (if we have it) of the asteroids being broken up by our weapons so they know its something we can handle. The one thing we want to avoid is people spreading rumor or trying to cause a panic or be doomsayers or anything like that.
We still need a lens in Gaian orbit to knock out any dust- or ice-balls that come our way, and a heavy torpedo to crack open the iron-nickel or dense asteroids. Run estimates on how many asteroids our system could reliably bring down and update it every time a new platform comes online, and see if we can use the telescope to make out the largest size for those asteroids the Moonies are gathering together. Find out where they are gathering them from if possible, watch the process in action -- they might be using a tractor-beam equipped vessel or similar device to shunt them in place. If so we can send stealthed combat patrols to hide among the asteroids and wait for them to show up; of course this runs a serious risk of us losing our ships, but it might be worth a shot and we do need a way to strike back. Our reflective stealth coating isn't doing anything else at the moment, anyway. Start with a few Minelayer Corvettes, send them up to any cluster of asteroids we think is close enough for them to go for yet not close enough to affect with their gravitic fields. Drop several combat drones at key positions and watch what happens.
Oh, and gravity manipulation is still *far* outside of our tech range; we're going to need a reliable source of fusion power to get that much energy and we need to figure out how to manipulate it in the first place. Really it might cost more than its worth right now, though I really love the idea of using the Moonies' own weapons against them.