Preheat oven to 375 degrees. Butter and flour two 8 1/2-by-4 1/2-inch (6-cup) loaf pans (see note, below); set aside. In a large bowl, whisk together flour, baking powder, ginger, and salt. In a medium bowl, whisk together sugars, pumpkin, melted butter, and eggs; add flour mixture, and stir until just combined.
Divide batter between prepared pans. Bake until a toothpick inserted in center of loaves comes out clean, about 50 minutes. Let cool 10 minutes; invert pans and transfer loaves to a wire rack to cool completely. Glaze, if desired.