Cyborganic possibility and Name origin[]
If you look closely at a greap's head you'll notice 2 tiny white arms perhaps greaps are small creatures but infused with their bomb-like body death scythes and wheels.
then again maybe the small arms are manipulators to handle small objects with the giant scythes (obviously) can't,
Also the name could be a combination of gruesome and creep,as scythes are associated with gruesome and they're creeps.