Cut the pineapple into spears or rings, removing the core. Super easy to just use a pineapple corer/slicer tool. Preheat gas grill to medium-high heat.
In a medium microwave-safe bowl, melt the butter, then whisk in the brown sugar and cinnamon. Add the pineapple to the bowl and gently coat with the mixture.
Place the pineapple pieces on the grill and cook for a few minutes on each side, or until grill marks begin to appear. Baste with sugar mixture during grilling. Depending on the size of the pineapple pieces, cooking time will be 5-10 minutes total.