Preheat oven to 400°F. Grease a shallow 9x13 baking dish with cooking spray or butter.
Layer one third of the sweet potatoes in the baking dish, slightly overlapping them. Sprinkle one third of the cheese on top of the potatoes. Repeat this process two more times to create three layers of potatoes and cheese.
Whisk together the cream, marinade, garlic and a large pinch of salt an pepper in a small bowl. Pour this mixture over the potatoes then gently shake the baking dish to make sure the cream is well distributed. Note: All of the liquid mixture may not be needed -- pour enough of it into the baking dish until it just reaches the top layer of potatoes, but does not cover it. Cut butter into small pieces and put it on the top of the gratin.
Cover dish with aluminum foil and bake for 30 minutes, then remove foil and bake for another 30 minutes or until the potatoes are tender and the top is golden-brown in spots. If the gratin is getting too brown on top but the potatoes aren't yet cooked through, place the foil back on top until the potatoes are done.
When cooked, remove from oven and let cool for at least 15-20 minutes before serving.