In the bowl of your food processor, add in GF flour, sugar and salt and pulse a few times to combine.
Add in the egg and cold cubed butter and process until the dough comes together in a ball. (Be patient before you add any liquid as it takes a minute or two to come together. If you find your dough is dry and crumbly, add in 1/2 tsp of ice water at a time until it forms a ball.)
Remove the dough from the bowl of the food processor and work it with your hands, just a little bit, to shape it into a round disk.
Wrap the dough in parchment paper or cling wrap and let it rest for at least 30 minutes in the fridge.
After 30 minutes, preheat oven to 350° F / Grease and flour a 10 inch tart pan, preferably with a removable bottom.
Transfer the dough onto a floured surface and roll it out into a circle big enough for your tart pan, sprinkling additional flour as needed, and turning the dough, to prevent it from sticking to the rolling pin and surface.
Transfer the rolled out crust to the tart pan; don't worry if some of it breaks as you're transferring it, as it is easy to push back together with your fingers and fill in any broken bits. Set aside.