Combine chicken and gravy in a small saucepan; heat thoroughly.
Beat egg yolks until thick and lemon colored. Stir in milk, salt, and pepper. Beat egg whites (at room temperature) until stiff peaks form; gently fold into yolk mixture.
Melt butter in a large ovenproof skillet over medium heat. Remove from heat. Pour egg mixture into hot skillet. Bake at 350° for 20 minutes or until knife inserted 1 inch from edge of skillet comes out clean.
Slide omelet onto a large warm serving platter. Spoon half of chicken mixture over omelet; fold omelet in half, and spoon remaining chicken mixture around sides. Garnish with parsley sprigs, if desired. Serve the omelet immediately.
Note: For well-seasoned gravy, use any prepared gravy mix or Roast Fowl Gravy.