Preheat oven to 400°. Melt butter in a large saucepan over medium heat; add garlic, and sauté 1 minute. Stir in salt, pepper, and 6 cups water. Increase heat to medium-high, and bring to a boil. Gradually whisk in grits. Reduce heat to low, and simmer, whisking often, 15 minutes or until thickened. Remove from heat; add cheese, and whisk until melted.
Whisk egg yolks until thick and pale. Gradually whisk about 1 cup of hot grits mixture into yolks; whisk yolk mixture into remaining hot grits mixture.
Beat egg whites and cream of tartar in a large bowl at high speed with an electric mixer until stiff peaks form. Fold one-fourth of egg whites into grits mixture. Fold grits mixture into remaining egg whites, and pour into a greased (with cooking spray) and floured 3-qt. baking dish.
Bake at 400° for 40 to 45 minutes or until puffed and golden brown. Serve immediately with Mushroom Gravy.