Preheat oven to 350 degrees. In a food processor, combine bread and Parmesan. Pulse until coarse crumbs form, about 3 to 4 times; set aside.
In a large saucepan with a lid, melt butter over medium heat. Add flour; cook, whisking constantly for 1 minute. Whisk in milk.
Add cauliflower, and season with salt and pepper. Bring to a boil, and reduce to a simmer. Cover, and cook until starting to soften, about 5 minutes. Remove from heat; gradually stir in Gruyere.
Pour mixture into a 2-quart baking dish, and sprinkle with breadcrumb mixture. Cover with aluminum foil; bake until cauliflower is easily pierced with a knife, about 20 minutes. Remove foil, and bake until breadcrumbs are golden brown, about 20 minutes more. Serve.