I am a little bit confused on how I should construct my .mtx file to be used in mri_glmfit. The following variables are what I have used in my fsgd file: 

GroupDescriptorFile 1,,,,
Title Study,,,,
Class B,,,,
Class NB,,,,
Variables ,,,Gender,Age
Input, Sub_ID,NB,F,71

Input, Sub_ID, B, F, 80 

Please let me know if you need more information on my work. Thank you! 

All the best, 

Alex Job