Preheat the oven to 425°F.
Place the cauliflower florets on a baking sheet. Drizzle with olive oil, then season with garlic powder, salt and pepper. Toss to coat, then roast in the oven for 20 minutes, turning at the halfway point.
Meanwhile, melt the butter in a small saucepan over low heat. Add the flour, stirring constantly with a whisk or wooden spoon until well blended. Slowly whisk in the warm milk and stir until the mixture thickens, about 3-5 minutes.
Remove from heat, season with nutmeg and mix in half the cheese.
Transfer the roasted cauliflower from a baking pan to an 8x11 casserole dish. Pour the sauce evenly on top. Then sprinkle with the remaining gruyere and panko breadcrumbs.
Low the oven temperature to 375°F and bake until the top is browned and bubbly, about 20-25 minutes.