I wouldn't add gravel over the roots and set a tub on it. If the tree is alive, it will continue to get bigger, and if the tree is dead, the roots will eventually rot and cause a sink hole. I think your only options are to cut the roots out, or build a raised deck like a did. If you've ever walked down a sidewalk where the trees in the terrace are now huge, the roots have heaved the walk up substantially. I'm working on Johnson St. in Madison right now, and the sidewalks are up and down because of the old trees. I used 2X8's, about 1' on center to support the tub. 4X4's at the corners down about 5'.