Prepare The Bread. If you have time, let the bread sit out at least a few hours up to a day in advance to get a little hard and stale. This helps the bread hold its shape! Dice the bread into 1-inch cubes. Transfer to a greased 9x13 pan and set aside. (I prefer a glass or ceramic pan for this recipe.)
Make The Custard. In a large bowl, combine pumpkin puree, milk, maple syrup, eggs, butter, vanilla, pumpkin spice, and salt. Whisk until well combined. The pumpkin spice may clump slightly, but you really want to make sure the eggs are smooth! (You can also use an immersion blender to get the mixture totally smooth, if you prefer.)
Pour the pumpkin custard mixture over the bread cubes, doing your best to coat the bread evenly. Use a flexible spatula to gently tuck any dry bread cubes into the custard.
Make The Streusel. If making the streusel topping, prepare it now. In a medium bowl, combine brown sugar, pecans, gluten-free flour, cinnamon, and cubes of butter. Use clean hands, a pastry blender, or two forks to work the butter into the dry ingredients until clusters form and no dry spots remain.
Rest. Cover the pan and let the custard soak into the bread for at least 1 hour, or cover the pan and chill several hours, up to overnight. Store the streusel separately until you're ready to bake.
Assemble & Bake. When you're ready to bake, preheat the oven to 350 degrees F. Break the streusel into clusters and sprinkle over the French toast casserole. Bake uncovered 40-45 minutes, or until the top is deep golden brown and the custard is just set. (Around the 30 minute mark, check on the casserole. If your streusel is browning too quickly, you can loosely tent the pan with foil to prevent burning!)
Serve & Store. Sprinkle the casserole with powdered sugar, or drizzle with pure maple syrup and enjoy! Cover leftovers and store in the fridge 2-3 days.