In 4-quart saucepan, melt butter over low heat. With whisk, stir in flour until smooth. Increase heat to medium; cook and stir until mixture is smooth and bubbly. Gradually stir in milk. Increase heat to medium-high; heat to boiling, stirring constantly. Boil and stir 1 minute; reduce heat to medium. Stir in Gruyère cheese. Cook until melted, stirring occasionally. Remove from heat; cool 5 minutes.