The Gaussian curvature is the product of the two principal [[MeanCurvature|curvatures]], so it is 1/mm^2.

see [[http://mathworld.wolfram.com/GaussianCurvature.html|Gaussian Curvature]]

also [[http://brainvis.wustl.edu/wiki/index.php/Folding/Measurements|Van Essen Lab Folding/Measurements]]