Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.

Bring chicken broth and salt to a boil in a large saucepan over high heat. Stir in grits, return to a simmer, then reduce heat to medium-low; continue cooking for 20 minutes, stirring frequently.

Meanwhile, melt margarine in a skillet over medium heat. Stir in green onions, bell pepper, and garlic; cook until peppers have softened, about 5 minutes. Stir in shrimp; cook until they begin to firm.

Stir shrimp mixture, tomatoes, Monterey Jack cheese, ¾ cup Cheddar cheese, and black pepper into grits; pour into the prepared baking dish. Sprinkle with remaining ¼ cup Cheddar cheese.

Bake in the preheated oven until cheese is bubbly and beginning to brown, 30 to 45 minutes.