A front end loader to carry in new gravel. For the rear end, at a minimum a box blade to tear it up and put it back together.
You might also want a landscape rake or a rear blade (get the model that TILTS and ANGLES. On gravel roads a lot of gravel moves to the sides, you can use the rake or the blade to move it back to the center and help recrown the road. Depending on your road use, drainage, etc, you might find that maintaining it with a rake a few times each season will reduce the real repairs you have to do with the box blade.