In the bowl of a stand mixer fitted with the paddle attachment, add the gluten-free pizza flour, sugar, instant yeast and salt. Mix to combine.
With the mixer running on low, slowly add the water and olive oil. Increase the speed to medium-high and mix for 5 minutes. The dough will resemble thick cookie dough, this is normal.
Using an oiled rubber spatula, scrape down the bottom and sides of the bowl and mix again until well-combined.
Generously oil the bottom and sides of a 10×14-inch Detroit pizza pan. Press the dough into an even layer all the way to the edges and corners of the pan. Cover the pan with plastic wrap and let rise for 90-120 minutes.
When the dough has about 30 minutes left to rise, place a pizza steel in the oven. Preheat the oven to 450°F.
Slide the pizza directly onto the pizza steel and bake (without toppings) for 10 minutes.
Meanwhile, mix together the preshredded cheese. Spread this cheese around the perimeter of the dough pressing it into the sides of the pan.
Sprinkle the hand-grated mozzarella cheese on the inside of the pizza. Pour lines of the tomato sauce over the cheese. Spread the pepperoni evenly over the pizza.
Bake the pizza for another 15-18 minutes, until the edges are caramelized and the interior is golden and bubbly.
Remove from the oven and let cool for 5-10 minutes before slicing.