Find a piece of scrap wood; a 2- by 4-inch piece roughly 12 inches long will suffice. Plywood is too thin for this task, but a 2-by-4 is ideal. If you don't have a length of wood handy, buy one at a store.
Divide the piece of wood into four squares using a tape measure. If using a 2-by-4, use a pencil to mark off four 4-inch square pieces.
Cut the wood into the four pieces using a circular saw. If you don't have access to a circular saw, a band saw, reciprocating saw or hand saw will suffice.
Note the location of the bench's feet in the gravel, then lift the bench out of the gravel. Clear away the gravel from these four locations and place the four pieces of wood firmly on the ground. If you can't find level ground below the gravel, consider shifting the position of the bench slightly to a spot where the ground is more level.
Set the bench back down so that its four feet sit on the blocks of wood. Rearrange the gravel around the bench's feet so that the blocks of wood are not visible.