gmc

Gun Motor Carriage
A type of military vehicle used mostly in WW2, that's lightly armored
and has a gun, that's useful as a support utility.
A self propelled artillery/AA gun/howitzer

Also used as other abbreviations, for example:
Genetically Modified Cabbage

Keep the GMCs behind the tanks!
The AAGMC in the convoy is there to provide mobile AA fire.
We need to dig in those GMCs before the counter attack or
they won't last long!