I'm new to chickens but not new to hot composting which is essentially what the deep litter method is, it *may* work just dumping the bedding ontop of the gravel depending on the gravel depth but you'd definitely have better results directly on the ground but I know moving gravel is a giant pain. If you feel it would be less work you can have topsoil delivered, dump that on top of the gravel and then start your deep bedding on top of that I've done that in the past, when I bought my house the previous owner had set the entire backyard to gravel with the intention of building a barn he never did, I wanted grass didn't want to move all the gravel, so I just covered it all in a like 1 inch layer of the cheapest topsoil and then put grass seed down and 1 to 2 years later it was a thick Green lawn just like the rest of the property so things will permeate through that gravel eventually if it's too much of a pain to move, it just might be a little bit slower to start