Preheat the oven to 375 degrees. Place the potatoes on a baking sheet lined with aluminum foil and bake for 45 minutes to an hour - or until potatoes are cooked through. Remove from oven and cut each potato in half. Let the potatoes cool until you can handle them, then scoop out the flesh into a bowl.
Use a potato ricer or hand mixer to puree the potatoes. Stir in the rosemary, salt and pepper. Add the milk and butter and stir until butter has melted. Add 1 1/4 cups of gruyere and stir to combine. Set aside.
Spray 4 half-cup ramekins with vegetable spray. Divide the sweet potatoes among the ramekins and smooth the tops with the back of a spoon. Sprinkle with the remaining cheese.
Can be made in advance to this point and refrigerated until ready to bake. To bake the gratins, preheat the oven to 350°. Place the gratins on a baking sheet and bake for 15 minutes until warmed through then turn on the broiler and cook for an additional 2-3 minutes until lightly browned. Serve.