Grist is an insect, or possibly the collective conscience of a swarm of insects.
Mark Rosewater supposes that whenever Grist planeswalks, she takes insects of the new world to create a body out of them.
Grist is referred to as she/her, likely indicating her status as the queen of the swarm she controls. Grist had access to black and greenmana.
The Song of Grist
The hunger tide sweeps across the land, calling us forth from slumber. A great noise shall rise from the earth as many mouths cry out for sustenance. Countless wings will darken the skies and famine's teeth shall strip the flesh from the bones of the world. Let all this and more come to pass, that our children may partake of the endless feast.