In a medium bowl, whisk together the flours, baking soda, spices, salt, xanthan gum, and baking power. Set aside.
In a separate bowl - the bowl of a stand mixer fit with a paddle attachment, or in a large bowl if you’re using an electric hand mixer - cream butter and sugars on high speed until fluffy.
Add molasses, eggs, and vanilla. Beat on medium speed until everything is fully incorporated and smooth - scraping down the sides of the bowl a few times, along the way. Add the milk, beat once again until smooth.
Turn your mixer down to low speed. Slowly add the dry ingredients mixture to the bowl of wet ingredients, carefully mixing until well incorporated and smooth.
Gather into a smooth ball, wrap tightly in plastic wrap. Chill dough for 1 hour.
Preheat the oven temperature to 350 F (180 C), line a couple of large baking sheets with parchment paper.
Drop heaping tablespoons of dough onto a parchment-lined baking sheet, roll into 1.5" cookie dough balls.
Dip 1 side of each gluten free cookie dough ball into sugar, and place sugar side up on a prepared baking sheet. Lightly press the tops of each to flatten slightly - just enough to prevent rolling around.
Be sure to leave at least 2" between cookies - they flatten out a fair amount.
Bake for 13-15 minutes. Cookies will be golden brown and look puffy, but flatten out as they cool. Do not over bake - take the baking time as a suggestion only!
Allow baked cookies to cool for a few minutes on the cookie sheet, before transferring to a wire rack to continue cooling.
Notes
Once cooled to room temperature, cookies can be stored in an airtight container for a week or so on the counter, longer if in the fridge.