Graynook is a level designed by Richard "Levelord" Gray in the leaked Blood alpha.
It's a tall castle map that has the player start way up in the castle, but eventually has the player reach the ground. There's a room filled with water, a small lake, and a tiny library inside the map.
The "gray" in the map name is Levelord's last name, Gray. The levels with "gray" in their names are older than the ones with "gm" in them.