Preheat the oven to 425°F 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 into a rectangle. Fold the rectangle into thirds and turn the dough a half turn. Then flatten into a rectangle until the dough is about an inch thick. Fold into thirds again and press into a rectangle again.
Use a sharp knife to cut the dough into squares and place the biscuits on the prepared baking sheet. (When cutting the biscuits, cut straight down and lift right back up without dragging the knife through the biscuits.) Re-roll the dough as needed; there should be about 8 biscuits. (If desired, you can cut the biscuits smaller to make up to 12 biscuits.)
Refrigerate the biscuits 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.