Please see the attached image.

It shows inflated fsaverage surface with gamma.mgh overlaid.

Gamma.mgh is from one contrast of a standard group analysis of cortical thickness (two groups A and B, two contrasts A>B and B>A).

1) Is it correct to interpret the gamma values as between-group difference of cortical thickness in mm? 

2) I would like to reproduce the histogram in another software. Is it possible to extract the gamma values for each vertex for this purpose?

All the best,

Anders