it stands for ground soldier
it is a gunner from a bomber that doesn't know the bomber has been killed yet (lag wise)
ever hear of the "tunnel of death" bug? a bomber gunner got stuck in an endless loop underground spinning around in the bomber unable to end flight.
when other people saw it you would see a GS zipping across the ground.
I also think it shows like that if the gunner bails out and is floating around.