Preheat oven to 375°F.
Cook gnocchi to al denté. according to package directions. Drain and set aside.
While pasta is cooking, prepare cheese sauce. Melt 3 tablespoons truffle butter in large saucepan or Dutch oven over medium heat. Add flour and whisk to combine, cooking over medium-low heat 3-4 minutes, whisking, until flour mixture becomes light brown in color. Add milk while whisking and cook over medium heat 5-10 minutes or until sauce thickens.
While the sauce cooks, melt remaining tablespoon of truffle butter. Add bread crumbs, 2 tablespoons parmesan cheese, melted butter, garlic clove and parsley to a food processor and process for 30 seconds or until minced. Set aside.
Remove sauce from heat and add swiss, white cheddar and remaining 1/2 cup parmesan cheeses; stir until melted. Season with kosher salt and white ground pepper.
Add drained gnocchi to sauce and stir to combine. Pour into an 8x8-inch baking pan or 1 quart casserole dish and sprinkle with bread crumb mixture and additional parmesan cheese, if desired. Bake 10-15 minutes or until the top is lightly browned and bubbly. Serve immediately.
Dish can be assembled before baking and refrigerated for up to 2 days. To heat, bake 35-45 minutes.