Depends on what you want to plot.
In each point (x,y,z), the gradient g=(gx,gy,gz) is a 3d-vector associated with this point.
But plotting 3d vectors attached to points in 3d-space won't give a reasonable plot in my opinion.
So without fixing a plane in which you want to see the gradient and maybe projecting the gradient onto this plane, you won't be able to visualize anything.
So you will first have to decide where and what you want to plot.
If you want to try a 3d-vector plot, use quiver3:
