Welcome Agent 007!
> -model "Time*Group+Age+Gender+(1|Subj)" \
With a longitudinal study, you may try to account for varying slope effect:
-model "Time*Group+Age+Gender+(Time|Subj)" \
> The effect of Group at Time 0 (?):
> -gltCode Group1-Group2 'Group : 1*Group1 -1*Group2' \
By default, 3dLMEr centers a quantitative variable around its overall average across all subjects. So, the above specification makes inference about group difference at the average time. For Time 0, try
-gltCode Group1-Group2 'Group : 1*Group1 -1*Group2 Time : 0' \
Your current model assumes a linear relationship of Time. In case nonlinearity is of interest, consider 3dMSS.
Gang