Toss sliced strawberries with 1/4 cup sugar in a medium bowl and let sit at room temperature for 30 minutes to macerate and release juices.

Preheat oven to 425°F (220°C) and line a baking sheet with parchment paper.

Whisk together the gluten-free flour, 1/4 cup sugar, baking powder, and salt in a large mixing bowl until fully combined.

Add the cold, cubed butter to the dry ingredients and use a pastry blender or two knives to cut the butter into the flour until it resembles coarse crumbs with pea-sized pieces.

In a separate small bowl, whisk together the milk, egg, and vanilla extract until emulsified.

Pour the wet ingredients into the flour mixture and stir gently with a spatula just until a soft, sticky dough forms; do not overmix.

Spoon six equal mounds of dough onto the prepared baking sheet, spaced at least 2 inches apart.

Bake for 12 to 15 minutes or until the shortcakes are firm to the touch and the tops are light golden brown.

While biscuits cool slightly, whip the heavy cream and powdered sugar in a chilled bowl until stiff peaks form.

Split each warm biscuit horizontally and layer the bottom half with macerated strawberries, followed by a dollop of whipped cream and the biscuit top.