IMT=Individual Marksmanship trainer.
Typical scenarios are like being in the range where uyou engage
static targets like canadian bull,Fig.11,Fig.12 etc.and basically
is a dry run to train your shooting skills before you advance to
the actual range using live ammunition.This is a safer way to train
recruits on marksmanship fundamentals and range drills etc.
SMT/GMT=Section Marksmanship Trainer /Group Marksmanship
Trainer
Basically a section will line up equipped with the weapons used in
a basic infantry section like the M16,Ultimax,M203,Armbrust etc.You
will be given a scenario with which to practice on be it a field
scenario or urban scenario.This allows the section to
communicate,and learn fields of fire etc on a given scenario,and
what weapons to employ against what targets.
Basically in IMT,you have lanes like in the range with which you
shoot at your assigned targets.In SMT, you will be free to engage
enemy screen-wide with whatever weapons assigned to you in a
section level.(hope it helps you understand)