Preheat oven to 350°. Grease then line bottom of 8-inch metal spring-form pan with parchment paper. Or use an oven-safe skillet.
TO MAKE DOUGH:
Add the flours, xantham gum, baking powder, Italian seasoning, garlic powder and salt to food processor. Pulse until dry ingredients are combined.
Pour in apple cider vinegar, pulse until evenly distributed. Pour in egg and pulse until combined.
Add water 1 tablespoon at a time, pulsing after each addition, until dough forms a ball. Dough will be sticky, but maintain the ball shape in your hand.
Wrap the ball of dough in plastic wrap and lightly knead with fingertips until smooth. If the dough will not hold together or has cracks in it (it's too dry) , return the ball to the processor and add more water 1 teaspoon at a time. Re-wrap in plastic and knead again until smooth.
Roll out the dough between sheets of parchment paper until roughly 10-inches round. Remove the top piece of parchment and transfer the dough to the pan by laying the dough, parchment side up over the pan.
Carefully remove the top layer of parchment. Gently push the dough down into the pan, smoothing the dough down into the bottom with your fingers and 1-inch up the sides of the pan. Fold any excess dough over the dough on the sides and pinch together.
Blind bake the dough 10 - 12 minutes on center rack of preheated oven until crust begins to turn golden brown in spots.
Remove crust from the oven and add toppings.
Sprinkle 1/3 of mozzarella over the crust in an even layer. Spread 1/2 of pizza sauce over the cheese. Add another 1/3 of the mozzarella over the sauce. Place 1/2 of the pepperoni slices over the cheese. Spread remaining sauce over the pepperoni, add remaining mozzarella cheese and then finishing with pepperoni on top.
Bake 20 -25 minutes on center rack of oven until cheese is melted and pepperoni is desired crisp.
After baking, place pan under broiler for 1 - 2 minutes if you like extra crispy cheese layer and pepperoni.
For this recipe, I used an 8-inch springform pan with removable bottom.