The problem with leveling a dip by adding simply gravel is that you then have a dip full of gravel which comes out rather quickly as traffic crosses it.
I have had a lot more success by breaking up the road surface to a depth of ~12", adding more material and compacting it all again. This way, the added material becomes part of the road surface as opposed to loose material dumped on the road.
I use the backhoe to scratch up the surface and then backdrag more material into the low spot, leaving a slightly high spot which will compact to a smooth road. I finish it off with a plate compactor, but compaction with the weight of the tractor front wheels (with a big load in the bucket) should be good enough.