Bring 2 cups water to a boil in a medium saucepan. Add the millet, and simmer, covered, until the water is absorbed, about 20 minutes.
Preheat the oven to 400 degrees F. Grease an 8-inch square baking dish.
Stir together the milk, garlic, 1 1/2 teaspoons salt and 1/4 teaspoon pepper in a small saucepan, and bring to a simmer over medium heat. Mix the cornstarch with 1 tablespoon water, stir the cornstarch slurry into the milk mixture and bring to a slight boil to thicken slightly. Stir in the corn, Gruyere and chives.
Place the millet in the prepared baking dish. Pour the corn mixture over it, and sprinkle with Gruyere, if using. Bake until all the liquid is absorbed and the cheese is melted, about 25 minutes.