Preheat your oven to 350°F (175°C). Grease the cake pan with cooking spray or butter.
Pour the melted butter into the prepared cake pan and spread it evenly. Sprinkle the brown sugar over the butter, then arrange the drained pineapple slices on top and place a maraschino cherry in the center of each slice.
In a medium bowl, whisk together the gluten-free flour blend, baking powder, baking soda, and salt. In a larger bowl, beat together the granulated sugar, eggs, and vanilla extract until smooth. Gradually add the flour mixture to the sugar mixture, alternating with the reserved pineapple juice and milk. Mix until just combined.
Pour the batter over the arranged pineapple and cherries in the cake pan and spread it evenly.
Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for about 10 minutes before inverting it onto a serving plate. Allow it to cool completely before serving.