Don't know how big your trailer is but you may be able to lay a tarp flat on the deck, dump the gravel on top of it, then pull the sides of the tarp up over the pile of gravel and tie it at the top basically creating a "bag" of gravel.
If only I had a brother in law with a dump trailer.
I built a "box" on my trailer (7 ft x 7 ft x 1 ft high) and lined it with a tarp. When I got the load to the house I used the bucket on the tractor to lift the tongue of the trailer high enough to dump it.