Use Cultures for Health Gluten Free Sourdough Starter, available on Amazon https://amzn.to/4c7iiIT. You will need to start feeding your starter in advance to have some discard to use in this recipe.

Add 2 cups gluten-free flour blend, 2 tablespoons baking powder, and 1 teaspoon sea salt to a large mixing bowl. Use a whisk to blend the dry ingredients.
You can cut the 7 tablespoons unsalted butter into the dry ingredients one of two ways. You can use a pastry cutter as I show above, or you can freeze the butter and use a cheese grater to grate the butter into the dry ingredients.
Add 1 cup gluten-free sourdough discard to your 1/2 cup milk or non-dairy milk am using oat milk in my test.) My discard became a little firm when stored in the refrigerator. It softened easily when I whisked it into the milk.
Pour the wet ingredients into the dry ingredients. Mix the ingredients into a nice, workable dough. The dough shouldn't be too sticky or dry.

Optional: Wrap the dough in plastic wrap and refrigerate the dough for 30-45 minutes.
Remove the dough and preheat the oven to 425º F. I also check to ensure the oven rack is positioned in the middle of the oven.
You can shape these biscuits one of two ways, in a circle with a biscuit cutter or cut them as squares (easiest!) For this recipe, I am going for easy! Dust a silicone mat with a little flour and place the dough on the mat.
Flatten your dough with your hands or a rolling pin, fold it over itself, and flatten it again three times. This helps give structure for layers to rise during baking. Try to do this quickly so the butter doesn't melt. Take a dough scraper and use it to cut the dough into biscuit squares. You can make them any size you like.

Place the dough squares onto a parchment paper-lined baking sheet. If you do not have parchment paper, you can use a gluten-free baking spray to grease your pan.
Brush the tops of each biscuit with milk. This helps the tops get nice and browned.
Bake the biscuits for 22-25 minutes. The actual baking time can vary depending on the size and thickness of your biscuit dough. You will know the biscuits are done baking when they are risen up and the tops are golden brown.
Removed the baked biscuits to a cooling rack.