Preheat the oven to 400 degrees F, ensuring one oven rack is in the center position & a second oven rack is in the top position under the broiler. Lightly spray a 9×13 deep baking dish with cooking spray & set aside. Gather & prep all ingredients according to ingredients list.
Place the bacon in a large skillet over medium heat. Cook until rendered & crisp, 4-5 minutes per side. Transfer cooked bacon to paper towel-lined plate & set aside. Carefully transfer 3 tablespoons of the bacon drippings from the skillet to a small bowl. Drain & discard any remaining bacon drippings.
Add flour to the small bowl with the reserved bacon drippings. Whisk to combine well. Transfer the flour mixture to the same skillet used to fry the bacon & place over medium heat. While whisking constantly, gradually pour the half & half into the skillet. Add the smoked gouda to the skillet, whisking until the cheese melts. Season with S&P as desired. The sauce may be a little lumpy – this is fine, it will smooth out as the au gratin bakes. Do not allow the sauce to come to a boil. Once melted, remove from the heat & set aside.
Transfer half of the sliced potatoes to the prepared baking dish, arranging them in a uniform layer. Spoon half of the prepared smoked gouda sauce over the potatoes. Repeat, creating a second layer of potatoes & finishing with the remaining smoked gouda sauce. Lightly spray one side of a piece of foil with nonstick cooking spray. Place the foil over the baking dish, oiled side down, wrapping the edges around the baking dish to seal.
Set the baking dish on a rimmed baking sheet & transfer to the oven. Bake 50 minutes, then carefully remove the foil & bake for 20 minutes longer, or until the potatoes are fork-tender.
Remove the baked potatoes au gratin from the oven. Place the sour cream over top, using a spatula to spread it into an even layer over the potatoes. Sprinkle the gruyère cheese over top. Preheat the broiler to its highest setting. Carefully transfer the loaded potatoes au gratin back into the oven, on the top rack under the broiler. Broil about 5 minutes, or until the gruyere is melted & golden brown. Remove from the oven.
Meanwhile, as the potatoes au gratin broils, finely chop the cooked bacon. Once the potatoes are removed from the oven, sprinkle the sliced bacon over top. Set aside to rest for 5 minutes – this helps the loaded potatoes au gratin set up & also warms the bacon through.