The values in the Surfer [.GRD] file are recognized as Component 1 (or "C") in Voxler. The “Z” value in a Surfer [.GRD] file can be any variable, (i.e. concentration of an ore body or pollutant), and not necessarily the Z value in XYZ space.
To display the Z values at the same scale as other objects, import the grid as a curvilinear lattice.
Updated October 10, 2016