Light a grill for direct medium-high heat, about 425°F . Mix the pomegranate molasses, brown sugar, butter, cardamom, cinnamon, nutmeg, and salt in a small bowl with a whisk until smooth. Using a sharp paring knife, remove the peel and bitter white pith from the grapefruits. Cut the fruit crosswise into 1-inch-thick slices, and then brush all over with the pomegranate molasses glaze. Brush the grill grate and coat with oil. Grill the grapefruit directly over the heat until nicely grill-marked, 2 to 3 minutes per side. Remove from the heat and brush with any remaining glaze.