How exact of an answer do you need?
It depends on the type of stone the gravel is made up of, and to a lesser extent on the size composition of the gravel. Granitic or limestone rocks are much less...
You do not because cubic means volume and square means area.
A tank that measures 1 yd wide by 1 yd long by 1 yd tall has a volume of 1 cubic yard and the area of its basis ( 1yd wide by 1yd long)...
There is no conversion between square yards and yards, because the units refer to different things. "Square yards" is a unit of area, while "yards" is a unit of length. But there is a relationship....
It depends on the size of your gravel (the amount of air trapped between the rocks). If you have small gravel (decomposed granite) it'll weigh more - if you have larger rocks, (river rocks, etc)...