Military doesn't "fix" it.
To clarify that and correct John.d.h:
Vanilla Glest can't.
MegaGlest probably can't either. (but I haven't really followed its development).
GAE can but I don't know how, only that there's a new option to put in the XML. You can find it out in Military though.
EDIT:
Correcting my correction:
What John.d.h said is perfectly correct because:
NONE of the above engines allows you to have a gun to independently rotate while the vehicle it's attached to is standing still (or is moving in another direction). GAE does have plans to implement this in the future through changes to the Pets feature.
Tracking a target is already available in GAE.