[Freesurfer] exctracting grey matter

Hi, I am using freesurfer for non_human primate. I did the segmentation in freesurfer , at the end i have the wm.mgz but not grey matter since the command to get it during segmentation doesn't work in monkey brain. in the forlder /surf/ i have all files such as lh/rh.pial, curv, sulc, white. Since I am also using SPM and some toolbox to conduct my connectivity data i need good segmentation, so i wanted to use the files in surf, the pial and white to get the grey matter information and then convert it to a volume file. I've been looking for solutions, but didn't find anything, can you help? how shall i proceed to get my grey matter mask from my segmentation, in the native space.

Thank you,

Best regards, Sherihane KUL