Prepare the Dough: In a large bowl, whisk together the gluten-free all-purpose flour, cornmeal, and salt.
Add the cold, cubed butter and use a pastry cutter or fork to cut it into the dry ingredients until you have a crumb-like texture.
Whisk the apple cider vinegar into the cold water, then pour gradually into the flour mixture, mixing until a shaggy dough forms.
Knead lightly 3-4 times, then wrap in plastic and chill for at least 20 minutes.
Prepare the Ricotta Filling: In a small bowl, whisk together the ricotta, parmesan cheese, basil, parsley, minced garlic, salt, and black pepper. Set aside.
Assemble the Galette: On a lightly floured surface, roll the dough into a 12-inch circle.
Spread the ricotta mixture evenly over the dough, leaving a 2-inch border.
Layer the heirloom and cherry tomato slices over the ricotta, drizzle with olive oil, and season with flaky salt and pepper.
Fold the dough over the filling to form a galette shape. Use a fork to press the edges and create a border.
Brush the dough with the egg wash.
Bake the Galette: Preheat oven to 400°F (200°C). Transfer the galette to a baking sheet lined with parchment paper.
Bake for 25-30 minutes until the crust is golden brown and the tomatoes begin to soften.
Let the galette cool slightly, then garnish with basil leaves before serving.