Depending on the level of infestation, the problem with weeds and grass growing in gravel roads is the grass blades and the roots make it harder to level the gravel after you bust it up. The grass will form clumps that make it hard to get a smooth level surface. If the weeds are well established, Mow them down as short as possible, then you will probably have to use the tines to bust the gravel up. Finally, if there are large clumps of roots you can go back and remove the larger clumps by hand. Then you should be able to get a smooth job of leveling.