To make Mornay Sauce: Heat butter. Add flour and stir in 2 cups milk. Cook until thickened. Season with salt and pepper. Whisk in 3/4 cream to thin sauce and add 1/3 cup grated Parmesan cheese.
To make Gnocchi: Melt 1/3 cup butter in 1 cup plus 2 tablespoons milk. When butter has melted, season with salt and pepper. Remove saucepan from heat and add in 1 cup all purpose flour all at once. Whisk until smooth. Return saucepan to heat and, with a wooden spoon, cook until the mixture sizzles and comes away from the sides of the pan. (You are making a panade). With a rubber spatula transfer to a baking pan and immediately cover surface with plastic wrap so that a skin does not form. Cool to room temperature. Transfer cooled mixture to a bowl and work in, one at a time, 4 large eggs. Do not add the next egg until the first is entirely absorbed. When all the four has been incorporated add 3 1/2 ounces grated Gruyere cheese.
To cook Gnocchi: Bring water to a boil. Fill a pastry bag, fitted with a 3/4 inch tip with gnocchi mixture. Rest metal tip of pastry bag on edge of pan and press the dough out. As 1/2 inch of gnocchi dough emerges, with a knife cut off dough and let it fall into the water. Do in batches of 20 or 30. Cook for 3 to 4 minutes, then remove gnocchi to a cloth towel with a slotted spoon.(gnocchi will deflate as they cool).
To bake: Preheat oven to 375 degrees. Put some Mornay on bottom of a shallow gratin dish and arrange gnocchi in one or, at most, two layers. Spread remaining sauce over top, spreading it evenly. Sprinkle top with 1/4 cup grated Gruyere and bake for 45 minutes or until gnocchi have swollen up again and absorbed sauce. Serve immediately before gnocchi deflate.