I think that will happen if the surface is considered planar already and the plane is parallel to the gumball orientation - in that case a “Z” scale makes no sense - no way to scale something that has thickness 0.
A right click in the Gumball status bar panel gets you the options. Otherwise, the following macros might help:
BTW, the image indicates that the surface - which should theoretically be a simple plane - has been “compromised” by some kind of editing that maybe at one point pulled it out of planar alignment, but then it got put back. You can see that in the density of the isocurves on the near edge. The easiest way to manually fix this is to select the surface in question and delete it, then re- Cap the object. Should make a nice planar replacement if the edges are OK.