You can also make this as one large gratin by baking it in a shallow two-quart dish for 40 minutes.
Preheat oven to 425 degrees. On a rimmed baking sheet, toss cauliflower with oil. Season with salt and pepper. Spread in a single layer and roast, turning once, until golden and crisp, 20 to 25 minutes.
Reduce oven temperature to 325 degrees. Melt butter in a medium saucepan over medium-high heat. Add flour and cook, stirring, 1 minute. Add milk, whisking until mixture just comes to a simmer. Gently simmer, stirring constantly, until mixture is thickened slightly, about 12 minutes. Remove from heat; stir in Parmigiano-Reggiano, nutmeg, and 1/2 cup Gruyere. Season with salt and pepper.
Divide cauliflower among six 8-ounce gratin dishes. Pour cream mixture over vegetables. Sprinkle with remaining 1 cup Gruyere.
Place gratin dishes on a rimmed baking sheet. Bake until bubbly and golden brown in spots, 23 to 25 minutes. Let cool slightly before serving.