Remove tree roots or whole tree?
But, if you're not worried if the tree stays or goes, then cut the roots, hope for the best, and make sure that the gravel base a little deeper and well compacted in that area, and pour your slab. Tree roots will seek the route that provides the best source of air and water. Growing under a slab isn't their first choice. They're likely to turn and seek another route, although there are no guarantees.