Preheat the oven to 425 degrees.
Line a baking sheet with parchment paper. Sprinkle the paper with some brown rice flour. Set this aside.
In the bowl of a food processor, add the brown rice flour, tapioca starch, xanthan gum, baking powder, baking soda, salt, sugar, and garlic powder. Pulse to combine.
Add the butter to the dry mixture and pulse until the butter is broken down into pea-sized chunks. Add the eggs and buttermilk, and run the food processor until the dough comes together in a ball.
Turn the dough out onto the parchment lined baking sheet, and flour your hands with more brown rice flour. Working quickly, pat the dough down into a 10-inch by 10-inch square shape, ¾-inch thick. Using a sharp knife, cut this square into 9 biscuits.
Gently rearrange the biscuits so they are not touching and have room to spread while they are baking. If you would like, gently brush the tops of the biscuits with milk, and sprinkle with course salt.
Bake the biscuits for 14-16 minutes, or until they are golden brown. Allow them to cool for 5 minutes on a cooling rack before serving. Store any remaining biscuits in an air-tight container.