In a large bowl, place the flour, (xanthan gum), baking powder and salt, and combine well. Add the chopped and chilled butter, and toss to coat it in the dry ingredients.

With clean hands, flatten each chunk of butter between your thumb and index finger. Add the yogurt (or sour cream), and fold in the dry ingredients with the yogurt. The dough should form into a somewhat crumbly blob. Press the loose flour into the blob until it begins to come together. 

Add ice water by the teaspoon only if necessary for the dough to hold together (I add about 1 teaspoonful). 

Form the dough into an elongated ball and wrap a sheet of plastic wrap around the dough. Place the dough in the refrigerator to chill for 30 minutes.

Preheat your oven to 375°F. 

Stir together the tapioca, vinegar, salt and coconut sugar until combined and the tapioca has formed a slurry with the other ingredients. Place the plumcot slices and raspberries into the mixture and stir until all pieces are coated. Set the bowl aside to allow the juices to be drawn out of the fruit while you roll out the pie crust.

Remove the galette crust blob from the refrigerator, slice it in half, and re-wrap and return one half to the refrigerator. Dust a 12 x 12 inch piece of parchment paper with gluten-free flour. Form the dough into a flattened disk about 4 inches across, place it onto the parchment paper, dust it with a little more flour, and cover it with another sheet of parchment. Roll it into a round about 10-inches in diameter or until it is about ⅛-inch thick and remove the top sheet of parchment.

Transfer the pie crust on its parchment paper to a rimmed baking sheet.

Leaving a 1 ½-inch border all around, arrange half of the fruit mixture in the center of the dough, piling the fruit pieces a bit toward the center. Fold the dough toward the center by placing your hand under the parchment paper at the edge of the galette and bringing the paper up and over, folding the dough over with it. This should result in small folds or pleats in the outer edges of the dough.

Sprinkle the galette with the coarse sugar if using.

Repeat with the second piece of galette crust and the remaining filling. Drizzle the remaining tapioca slurry over the middle of each galette. 

Place the baking sheets in the preheated oven. Bake until the fruits are softened and the crust is lightly golden brown, about 35 minutes. Remove the galettes from the oven and allow them to cool on the baking sheet until the filling is mostly set. Place coconut yogurt (or sour cream) on the table for garnish.