Grits (the food) originated with Native American tribes in what is now the Southern part of the US.Since he is talking about the gizzard, I think this "grits" must be the plural of grit (little stones for the gizzard to grind food with.)
I know that "grits" can be a kind of food, but I do not think that makes sense here. (Unless he thinks that porridge is somehow good for the gizzard, unlike all the other foods he is mentioning.)
I don’t know when this chap was writing or where he was from but if it isn’t Southern US I suspect grits for him was the plural of grit meaning small stones.