"Gradient Bump" is only for the old P4 .bum bumpmaps, and rendering with the P4 render engine. At least, that's what the manual says. One of the problems with white (or black) is that you don't have any room for the render engine to add highlights (or shadows). I'd take the base colour on the diffuse node down a step. Also, the highlight size can make a difference. Alternative Specular, and the different types of reflection shader, can make a difference, but it's not something I've ever fiddled with. I am led to understand that DAZ have officially dropped the requirement for P4 support, but on what I've seen I think they are still struggling a bit to do more than convert a P4 textureset to P5.