Preheat the oven to 400 degrees F. Line a baking sheet with foil and lay the bacon out on top. Bake until crispy, 15 to 20 minutes.
Meanwhile, cut off the green leaves from the cauliflower. Using your hands or a knife, cut the florets away from the core.
Remove the bacon from the baking sheet and set aside. Put the cauliflower on the baking sheet and, using a heatproof spatula, toss with the bacon fat left behind. Spread out in an even layer, sprinkle with 1/4 teaspoon salt and some pepper and roast until golden brown, about 20 minutes.
Meanwhile, in a saucepan over medium heat, melt the butter. Add the flour and cook, whisking constantly, until a thick paste forms and turns golden brown, about 2 minutes. Add the milk 1/2-cup at a time, whisking well between each addition, and bring to a bare simmer. Whisk in two-thirds of the smoked Gouda until melted. If the sauce is too thick, add more milk, 1/4-cup at a time, until the desired consistency is reached.
Place the roasted cauliflower in 13-by-9-inch baking dish or large oval casserole. Crumble the bacon over the cauliflower, then pour the cheese sauce over the top, making sure all the florets are covered. Sprinkle with remaining Gouda and bake until the cheese is bubbling and the top is golden brown, 30 minutes.