You can just do a bechamel with cheese. You melt butter, whisk in flour, then warm milk until thickened and stir cheese in at the end. I don't usually use a recipe, but Larousse uses 3 T butter, 6 T flour and 2 cups milk (which has been steeped with onion, bay leaf and mace). Add a little nutmeg and about 3 oz or so of your cheese of choice. I find that creamy enough but you could certainly stir some cream in as well. Or take it up a fatty-licious notch and make a mornay. Silver Spoon calls for blending a yolk with 1/2 c cream. Stir 1 cup each swiss and parmesan into the hot bechamel sauce then add the yolk mixture.

For a creamy gnocchi sauce, I'd probably do a simple 'alla panna' (instead of a bechamel, with flour.) Really simple - just melt a couple tablespoons of butter, add about a cup of cream, pinch of nutmeg and cayenne, and simmer until it thickens a bit. Stir in some grated parm, and s&p if needed. (Peas and/or sauteed mushrooms are also nice with a pasta alla panna.) Very rich, but the gnocchi only need a light coating, not to be swimming in sauce. Serve more grated parm on the side...delicious.