In a food processor, add gluten-free flour, xanthan gum, cinnamon, nutmeg, salt and brown sugar and pulse to combine.
Add vegan butter and pulse until butter is the size of peas, with some bigger pieces.
Remove from food processor and place in a bowl, add in water one tablespoon at a time just until a dough forms. You may not end up using all of the water.
Pat into a flat circle and chill for at least 30 minutes.
To make the filling, combine diced rhubarb, peaches, blueberries, cornstarch, sugar, and lemon juice in a bowl and mix together. Allow to sit for 5-10 minutes. Filling should be slightly thickened.
Preheat oven to 375F. Line a baking sheet with parchment paper.
Roll out the dough to a 12-inch circle about 1/8 inch thick.
Spoon filling into the middle of the dough.
Crimp dough over the filling leaving a circle in the middle. Brush crust with melted butter and sprinkle turbinado sugar along the edges.
Bake at 375F for 40-50 minutes, until crust is golden brown.