> Could you please let me know if this is the Group by Time effect I am looking for?
Yes, that's the conventional omnibus interaction effect between Group and Time in your case. It can be better understood when combined with the four partitions you specified:
-gltLabel 1 'Apost-Apre' -gltCode 1 'Group : 1*A Time : 1*post -1*pre' \
-gltLabel 2 'Bpost-Bpre' -gltCode 2 'Group : 1*B Time : 1*post -1*pre' \
-gltLabel 3 'Apre-Bpre' -gltCode 3 'Group : 1*A -1*B Time : 1*pre' \
-gltLabel 4 'Apost-Bpost' -gltCode 4 'Group : 1*A -1*B Time : 1*post' \
Gang