Use a spoon to scoop out the flesh, leaving a ½-inch border all the way around. Mash with a large fork and stir in the butter until it is melted. Add the salt, cinnamon, nutmeg, coriander, dried sage, and pepper and stir well. Place the mixture back into the squash shells and sprinkle with the Gruyere. Return to the oven until the cheese is melted, about 3-5 minutes.