Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper. Spread the cauliflower florets in a single layer.
Drizzle with olive oil and toss to coat. Roast for 18 minutes or until lightly browned and tender.
Meanwhile, prepare the cheese sauce by heating the cream in a saucepan over medium heat. Stir frequently until it simmers.
Add the cream cheese and whisk until melted and fully incorporated.
Reduce heat to low. Stir in ¾ cup of cheddar cheese and all the gouda. Continue stirring until the cheeses melt.
Season the sauce with garlic powder, mustard, salt, black pepper, and cayenne if using. Cook for 3–4 more minutes until smooth and thick.
Add roasted cauliflower to the sauce and toss gently to coat.
Transfer to a 9x9 casserole dish and sprinkle the remaining cheddar cheese on top.
Bake for 20 minutes. For a golden, crispy top, place under the broiler for 2–3 minutes, watching closely.