Preheat your oven to 350°F (175°C). Grease a 9x5 inch loaf pan and line it with parchment paper.
In a medium bowl, whisk together the gluten free baking flour, gluten free oats, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.
In a large bowl, beat the softened butter, dark brown sugar, and granulated sugar together until light and fluffy, about 3 minutes. Beat in the eggs one at a time, then mix in the vanilla extract and milk until smooth.
Fold the dry ingredients into the wet ingredients with a spatula until just combined. The batter will be thick with a gorgeous oatmeal cookie aroma.
In a small bowl, beat together the cream cheese, powdered sugar, vanilla extract, and marshmallow fluff until completely smooth and creamy.
Pour half the bread batter into the prepared pan. Drop generous spoonfuls of the cream cheese filling across the surface, then add the remaining batter on top. Use a toothpick or skewer to create swirls through the batter, pulling through the cream cheese layer in a zigzag pattern.
Bake for 55 to 65 minutes, until the top is golden brown and set, and a toothpick inserted near the center comes out clean or with a few moist crumbs. The cream cheese swirl area may still look slightly soft, which is perfect.
Let the bread cool in the pan for 20 minutes before removing. This bread is incredible sliced thick while still slightly warm.