Preheat oven to 425 degrees and line a baking sheet with a silicone baking mat or parchment paper.

Add the gluten-free flour, xanthan gum, salt, baking powder, and baking soda to a large bowl and whisk together to combine.

Cut in the cubed butter with a pastry cutter or two forks until it resembles coarse crumbs.
Add the chopped sage and grated cheddar and stir to combine. Stir in ¾ cup buttermilk and mix until incorporated.

Turn the dough out onto a floured surface and pat it into a rectangle. Fold the rectangle into 3rds and turn the dough a half turn. Then flatten into a rectangle until the dough is about an inch thick. Fold into 3rds again and gently roll into a rectangle again.

Use a sharp knife to cut the dough into squares and place the biscuits on the prepared baking sheet. Cut into 8 large or 12 smaller biscuits.

Refrigerate the biscuits for at least 30 minutes to ensure that the butter stays nice and cold. This is especially important if your house/kitchen is warm and humid.

Brush the tops of the biscuits with the remaining buttermilk. Bake for 12-15 minutes or until the biscuits are browned and baked through. Serve warm out of the oven.