change something in the hier file and see which one is affected, or look at the TGA files and see which one matches which. These are quite easy things to figure out. In any event, I think T gun is for Turret (or tail) gunner, and B gun is for Belly gunner.
N gun is usually for Nose gunner, L gun is for left side gunner, R gun for Right side, F gunner is mostly for Front gunner (if it is not called N gunner) and A gunner for Ass gunner.