Preheat oven to 350 degrees and line baking sheets with parchment paper or silpat. Set aside.
In a large mixing bowl or stand mixer, beat together molasses, brown sugar, granulated sugar, eggs, and butter until pale and smooth. Add vanilla and mix until incorporated. Add ginger, cinnamon, nutmeg, salt, baking soda, and xanthan gum. Mix until combined. Add brown rice flour, oat flour, white rice flour, and tapioca starch. Mix until incorporated. Mix an additional 2 minutes or until dough begins to pull away from sides of the bowl.
Cover dough and chill in fridge for at least 30 minutes or until dough is firm enough to be handled.
Roll dough into walnut sized balls and place on cookie sheets 2 inches apart. Flatten dough with hand to 1/4? thickness. Bake at 350 degrees for 8-9 minutes or until cookies are browning along edges. Allow to cool for 5 minutes before transferring to wire racks to continue cooling. Continue baking one pan at a time until all dough in gone.
Once cookies are completely cooled, sprinkle tops with powdered sugar. Store in airtight containers for up to 5 days or freeze for up to 3 months.