Jordan,
Even though you dummy code the variable Group in both of your scripts, but you didn't declare it as a quantitative variable. Therefore 3dLME simply treated it as a factor, and that's why you got exactly the same result between the two approaches. To let 3dLME do what you intended to achieve, declare Group as a quantitative varialbe *and* center it properly with something like:
3dLME -prefix lme-B+C \
-jobs 4 \
-model 'Group*Age' \
-qVars 'Group,Age' \
-qVarCenters '0,14.79' \
-ranEff '~1|Subj' \
-resid lme-B+C-resid \
Gang
