Preheat oven to 350°F. Grease an 8-inch square pan and set aside.
Pour boiling water over oats, cover and let stand for 20 minutes.
Beat butter, gradually adding sugars, until fluffy. Blend in vanilla and eggs. Add oat mixture and mix well.
Mix together in another bowl flour, xanthan gum, salt, baking soda, cinnamon and nutmeg. Add to creamed mixture and mix well. Pour batter into prepared pan. Bake for 50 to 55 minutes.
Combine all frosting ingredients. Spread evenly over still-warm cake. Broil until frosting bubbles. Makes 16 servings.