Preheat the oven to 350°F. Spray an 8-inch cake pan with cooking spray.
In a large bowl, whisk together the gluten-free flour, sugar, baking powder, baking soda, kosher salt, cinnamon and nutmeg
Add the sliced apple to the dry ingredients and stir briefly to combine.
In a medium bowl, whisk together the yogurt, oil, eggs and vanilla until smooth.
Add the wet ingredients to the dry ingredients and gently mix with a rubber spatula until combined. Pour the batter into the prepared cake pan.
FOR THE CINNAMON TOPPING: Make the topping by combining the brown sugar, cinnamon, butter and salt. Gently mix with a fork, keeping the mixture crumbly. Do not over-mix. Sprinkle the topping evenly over the cake batter
Bake the cake for 30-35 minutes, until golden brown and a toothpick inserted into the center comes out clean.
Remove from the oven and cool in the pan for 10-15 minutes. Remove the cake from the pan and transfer to a wire rack to cool completely.
FOR THE GLAZE: Once the cake is fully cooled, make the glaze. In a small bowl, mix together the powdered sugar and 1 tablespoon of milk. Stir until smooth. If the glaze is too thick, add an extra tablespoon of milk. Drizzle the glaze over the cake.