Ian,
You can use the mris_anatomical_stats command with the -b option. E.g.,
mris_anatomical_stats -b subjectid lh
will give you a tabular output containing the total gray matter volume in the 3rd column:
128449 86239 243375 2.588 0.876 0.123 0.026 1496
133.3 /xxx./subjects/QEN001/surf/lh.white
Xiaojiang