Preheat oven to 400°F. In a food processor, pulse gluten free flour, xanthan gum, baking powder, and salt. Add cold butter and pulse until mixture resembles coarse crumbs.
Add cold water and pulse until dough comes together. If too dry, add water 1 tablespoon at a time.
Roll out dough between two sheets of parchment paper until very thin, about 1/8 inch thick. Brush with beaten egg and sprinkle generously with everything bagel seasoning.
Cut into squares or rectangles and transfer on parchment to a baking sheet.
Bake for 12-15 minutes until golden and crispy. Cool completely on a wire rack. Store in an airtight container for up to 5 days.