In a bowl, combine sourdough starter, 2 cups gluten free flour blend, and water. Mix well. This dough will be somewhat wet.
Allow rising 4-6 hours. (I typically rise overnight, or through the day so it is ready for pizza night.)
Mix additional 1 cup flour, baking soda, and salt in a small bowl.
Add this mixture to the dough and mix.
Sprinkle some flour onto the parchment paper and spread to cover. Dump the dough onto a parchment paper-lined baking sheet.
Dust the top with additional flour and press the dough flat. You may need up to ½ cup additional flour for this process. As you press the dough, sprinkle the top with flour if the dough is sticky. Press the dough until it is ¼ inch thickness.
Allow the crust to rise for 45 minutes. Preheat your oven to 425º F.
Pre-bake your crust for 10 minutes. Remove from the oven.
Add your pizza sauce, cheese, and toppings.
Return the pizza to the oven and bake an additional 10-12 minutes until the cheese is slightly golden.