Stir together the gluten-free flour, sugar and salt until evenly blended, then put in the bowl of a food processor. Add the butter and pulse until coarse and uniform. Transfer to a mixing bowl.
In a small bowl, whisk together the egg and buttermilk. Gradually add the egg mixture to the flour mixture, stirring with heavy duty scraper to make a dough. Shape into a ball. Wrap in plastic wrap and chill for 30 minutes. Alternatively, you can press it into a flat disk (which is what I do) which makes rolling a little easier.
On a lightly floured pastry mat, roll into a 12 inch circle. Use a benchscraper or pie lifter to transfer the dough to a 9 inch pie dish. Fit the dough into the dish and crimp the edges. This is a good time to begin preheating the oven.
Freeze the pastry dough just until firm (this protects the crimped edges from getting squashed), then press a sheet of parchment paper over it. Make sure the parchment comes out over the edges to shield, then and weigh down with pie weights or beans.
Preheat oven 425 degrees F if you haven't already. Set the pie crust with the foil and pie weights on a rimmed baking sheet and bake for 15 minutes. Remove foil and pie weights. Edges should be just starting to brown. Reduce heat to 375 and bake for another 10 to 15 minutes. Check at 10. The trick is to bake it through and through without it getting to brown, so if edges are browning too quickly, lay a piece of foil loosely over.