Preheat your oven to 350°F. Place the unsalted butter in a 9-inch round cake pan and set it in the oven for a few minutes until the butter melts. Remove the pan, tilt to coat the bottom evenly, then sprinkle the light brown sugar in an even layer over the melted butter.
Arrange the pineapple rings in a single layer over the brown sugar, fitting them snugly. Place a maraschino cherry in the center of each ring and in any gaps. Set the pan aside.
In a large bowl, whisk together the gluten free 1-to-1 baking flour, granulated sugar, baking powder, and salt.
In a separate bowl, whisk together the vegetable oil, eggs, whole milk, pineapple juice, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir with a spatula until just combined and no streaks of flour remain. Do not overmix.
Pour the batter gently over the pineapple arrangement in the pan, spreading it into an even layer.
Bake for 38-43 minutes, or until the top is golden and a toothpick inserted in the center comes out clean. Let the cake cool in the pan for exactly 5 minutes, then place a serving plate on top and carefully flip it over. Lift the pan away and serve warm or at room temperature.