You will need a 2 part bumper filler. The paint stores should have what you want. It is not like regular body filler(bondo). It is more like JB weld. You can get it in 2 tubes or a single syringe type deal that is used with a caulking gun. The brand I use is made by SEM. This is the one is use for bumper cuts and gouges :
http://www.sem.ws/product.php?product_id=123