Combine first 4 ingredients in a medium bowl; make a well in center of mixture. Combine buttermilk, 1/4 cup egg substitute, and green chiles; add buttermilk mixture to dry ingredients, stirring just until dry ingredients are moistened. Spoon batter into a preheated 8-inch cast-iron skillet coated with vegetable cooking spray. Bake at 425° for 15 minutes or until golden. Remove cornbread from skillet, and cool slightly on a wire rack.
Crumble cornbread into a large bowl. Add turkey and next 9 ingredients, stirring well. Spoon mixture into a 13- x 9- x 2-inch baking dish coated with cooking spray. Bake, uncovered, at 350° for 1 hour. Cover and bake 30 additional minutes.
Melt margarine in a medium saucepan over medium heat; add flour, and stir until smooth. Cook, stirring constantly, 1 minute. Gradually add milk and chicken broth; cook, stirring constantly, until mixture is thickened. Remove from heat; stir in 1/4 teaspoon salt and 1/8 teaspoon pepper. Serve gravy with casserole.