Line a baking sheet with parchment paper and set aside.
In a large bowl combine the gluten-free flour mix, baking powder, baking soda and salt and whisk to combine.
Add the butter to the flour mixture and using your hands or a pastry cutter, smash the butter into the flour until chickpea sized balls start to form.
Mix together the buttermilk and egg until homogenous, then stream into the flour and butter mixture.
Use a spatula to mix together until a dough starts to form, then place the bowl of dough in the fridge for 20-30 minutes to chill the butter and let the dough rest.
Turn the dough out onto a lightly floured work surface and shape the dough into a 1 inch thick square.
Cut the dough into 4 equal pieces and stack them on top of one another, then gently compress the layers and shape the dough into a 1-1½ inch thick square.
Cut the dough into a 3x3 grid, then transfer the biscuit squares to the prepared baking sheet and place in the freezer for 15 minutes to firm up the butter.
Heat the oven to 400F.
Brush the biscuit tops with buttermilk and bake for 20-25 minutes or until golden brown and transfer to a wire rack to cool for 10 minutes.
While the biscuits are baking, make the gravy.
In a frying pan set over medium-high heat, brown the sausage until cooked through and slightly crispy.
Reduce the heat to medium and add butter to the pan.
Once the butter has melted, sprinkle the gluten-free flour mix over everything in the pan and stir to combine.
Cook for 1-2 minutes until the butter and flour form a blonde pan roux.
Whisking constantly, add the milk to the pan in ¼ cup splashes, making sure the milk is smooth before adding in the next splash.
Repeat until all the milk is used, then season to taste with salt and pepper.
Reduce the heat to medium-low and let the gravy simmer for 10-15 minutes until thickened to your preferred consistency.
Spoon over the biscuits and serve while warm.