Bring a large pot of water to a boil and cook the cauliflower for about 5 minutes. Drain and set aside.
In a medium saucepan, melt 2 tablespoons of the butter over low heat. Whisk in the flour until well combined with the butter and it forms a brown paste. Pour in the hot half-and-half and keep whisking until it starts to thicken. Remove from the heat and season with salt and white pepper. Add in the Parmesan and 3/4 cup of the Gruyere. Keep whisking.
Spread about 1/2 cup of the mixture on the bottom of an 8-by-8-inch baking dish. Place the cauliflower on top and spoon the rest of the sauce on top. Melt the remaining 2 tablespoons butter in a small saucepan over low heat and stir in the parsley. Drizzle the parsley butter over the cauliflower and sprinkle with the remaining 1/4 cup Gruyere.
Bake until the top of the gratin is browned, about 30 minutes. (My advice would be to check halfway through, as ovens vary.) Serve.