Preheat oven to 400 degrees. Heat 1/2 cup water, the butter, sugar, and salt in a medium saucepan over medium-high heat, stirring occasionally, until butter melts and mixture boils. Remove from heat, and stir in flour with a wooden spoon. Return pan to medium heat, and cook, stirring, until mixture pulls away from side of pan and forms a film on bottom, about 4 minutes.
Transfer batter to a bowl, and beat with a mixer on low speed until slightly cooled, about 2 minutes. Raise speed to medium, and add eggs, 1 at a time, beating after each addition. Beat 1 minute more. Batter should be shiny and form a string when pulled up with a finger; if string doesn't form, add water, 1 teaspoon at a time, until it does. Stir in Parmesan.
Transfer batter to a pastry bag fitted with a 1/2-inch plain tip (such as Ateco #806). Pipe about 30 mounds (1 inch in diameter) 1 inch apart onto parchment-lined baking sheets. Brush with egg yolk, and sprinkle each with about 1/2 teaspoon Gruyere.
Bake until gougeres are puffed and lightly golden, about 10 minutes. Reduce oven temperature to 350 degrees, and bake until golden and a toothpick inserted into the centers comes out dry, 20 to 25 minutes more. Serve warm or at room temperature.