Put the grits in a large bowl, fill with water and stir. Let the grits stand for about 5 minutes for the bran to rise to the surface; pour off the bran and excess water and set aside.
Bring the stock and half-and-half to a boil, over high heat, in a medium heavy-bottomed pan and slowly whisk in the grits. Reduce the heat to medium-low and cook, stirring often, until thick and creamy, about 45 minutes. If the grits get too thick, stir in additional hot stock or water to loosen and thin.
Add the Gruyere and the bacon drippings, if using, and stir until the cheese melts. Season with salt and pepper, to taste. Transfer to a serving bowl and serve immediately.