Make the crust by whisking together the gluten free flour, almond flour, xanthan gum, sugar, and salt. Toss in the cold butter and work it in with a pastry cutter or your fingers until you have a shaggy mixture with pea-sized butter pieces throughout.
Drizzle in the ice water one tablespoon at a time, stirring with a fork until the dough just holds together. Shape into a disc, wrap in plastic, and refrigerate for at least 1 hour.
While the dough chills, toss the strawberries and rhubarb with the granulated sugar, cornstarch, lemon juice, and vanilla. Let the mixture macerate for 15 minutes.
Preheat your oven to 400 F. Roll the chilled dough out between two sheets of parchment paper into a rough 13-inch circle. Transfer the dough with the bottom parchment to a large baking sheet.
Pile the fruit filling in the center of the dough, leaving about a 2-inch border all around. Leave behind any excess liquid in the bowl. Fold the edges of the dough up and over the outer ring of fruit, pleating as you go. The center should remain open.
Brush the folded edges with beaten egg and sprinkle the turbinado sugar generously over the crust.
Bake for 35 to 40 minutes until the crust is deeply golden brown and the fruit is bubbling. Let the galette cool on the pan for at least 20 minutes so the juices thicken. Serve warm with vanilla ice cream.