Preheat oven to 400°F (200°C). Wash and dry each stalk of Swiss chard. Remove the ends of Swiss chard stalks if dirty and then chop each stalk into thirds. Arrange Swiss chard in a large baking dish. Pour béchamel sauce over the Swiss chard. Top with grated Gruyère, salt, and pepper and bake for 20 minutes or until golden brown.