Preheat oven to 400F.
In a small bowl, rub grapefruit zest into granulated sugar until the fragrances are released and the zest and sugar are thoroughly combined.
In a food processor, add the sugar mixture, gluten-free flour, baking powder, and salt. Pulse to combine.
Then, add in the very cold butter chunks and pulse until the butter is the size of small peas. If you don't have a food processor you can also do this in a bowl with a pastry blender or two knives. You just want the butter to remain cold and not melt into the flour mixture.
Then, add in the vanilla and grapefruit juice. Start with ⅛ cup of grapefruit juice and keep adding more until the mixture is slightly wet and can be held together into a dough (I ended up using almost the full 1/2 cup of juice but it will depend on your brand of flour).
Carefully stir in the fresh grapefruit segments.
Pat the dough into a circle (about 8 inch in diameter). Cut it into 8 wedges.
Bake at 400F for 15-17 minutes, until they are a light golden brown.
Make the glaze while the scones are in the oven. In a small bowl, combine grapefruit juice, zest and powdered sugar and whisk until there are no more clumps of powdered sugar. You can add more juice if you want a thinner glaze.
Remove the scones from the oven and allow to cool before glazing.
Drizzle or spoon the glaze over the scones and allow to set completely before serving.