Are there significant differences between male and female GM values?  My understanding of the total covariate vs. separate covariate use depends upon your research question and the possible presence of significant differences between groups in those covariates.  Also, I would recommend mean centering the covariates, because not doing so may make it difficult to interpret the results.  There is really no such thing as zero GM volume (uncentered situation).  You would want to control for the average GM volume in your sample (centered situation).  

Hope this helps,
Jeff