Bring 4 cups chicken broth and 1/2 teaspoon salt to a boil in a large saucepan; stir in grits. Cover, reduce heat, and simmer 20 minutes.
Stir together grits, 3/4 cup Cheddar cheese, and Monterey Jack cheese.
Melt butter in a large skillet over medium heat; add green onions, bell pepper, and garlic, and sauté 5 minutes or until tender.
Stir together green onion mixture, grits mixture, shrimp, and next 3 ingredients. Pour into a lightly greased 2-quart baking dish. Sprinkle top with remaining 1/4 cup shredded Cheddar cheese.
Bake at 350° for 30 to 45 minutes.