In a medium bowl, mix together the gluten-free flour, cornstarch, salt, baking soda, baking powder, cinnamon, and pumpkin pie spice. Set aside.
In a large bowl, add the oil, granulated sugar, brown sugar, pumpkin purée, eggs, milk, Greek yogurt, vanilla, and whisk until smooth.
Add the dry ingredients and mix until combined. Take a silicone spatula and run it along the sides and bottom of the bowl to make sure you have no pockets of dry flour. Let the batter rest for 20 minutes.
While the batter rests, preheat your oven to 410°F (210°C) and line a muffin pan with paper liners.
Streusel
In a bowl, add the gluten-free flour, granulated sugar, salt, and cold butter. Use your fingertips to pinch and work the butter into the flour and sugar. Add the pepitas and mix.
Baking
Evenly portion out the muffin batter in the muffin pan, and top them with equal amounts of streusel (about a heaped teaspoon per muffin). Bake the muffins in the preheated oven for 7 minutes, then turn the temperature down to 375°F (190°C) without opening the oven door, and bake for an additional 16-18 minutes. A toothpick inserted into the center should come out clean or with a couple of crumbs on it. Let them cool for 10 minutes in the pan, and then move them onto a wire rack to cool completely.