For the crumb topping: In a medium bowl, mix together the brown sugar, granulated sugar, cinnamon and salt. Add the melted butter and mix to combine.
Add the gluten-free flour and use a fork to mix until large crumbs form. Do not use a spoon or overmix, or the mixture will become pasty. Cover with plastic wrap.
For the pumpkin cake: Preheat the oven to 350°F. Line a 9×9-inch baking pan with parchment paper and spray with cooking spray.
In a large bowl, whisk together the gluten-free flour, brown sugar, granulated sugar, baking soda, baking powder, salt, cinnamon, nutmeg and cloves.
In a medium bowl, whisk together the eggs, pumpkin, milk, oil and vanilla.
Add the wet ingredients to the dry ingredients and stir with a rubber spatula until just combined and no dry flour streaks remain.
Pour the cake batter into the prepared pan and smooth the top. Scatter the crumb topping evenly over the cake, keeping some of the crumbs in larger pieces.
Bake for 35–40 minutes, until a toothpick inserted into the center of the cake comes out clean.
Remove from the oven and let the cake cool completely before topping with the glaze.
For the glaze: In a medium bowl, whisk together the powdered sugar, milk, vanilla and salt until smooth. Add more powdered sugar if the glaze is too thin or more milk if the glaze is too thick.
Drizzle the glaze over the cooled cake and slice into squares.