Preheat oven to 350°. Grease a deep square pan or a 2-3 qt casserole dish.
In a large bowl, whisk together all of the filling ingredients; pour into the prepared dish.
In a medium bowl, combine the gingersnap cookie flour, oats, brown sugar, soda and powder. (To make gingersnap cookie "flour," simply use a food processor or blender to break down gingersnap cookies into a flour-like texture.) Then, use a fork to incorporate the butter into the dry mixture.
Evenly sprinkle the topping over the pumpkin filling.
Bake for 35-45 minutes, or until the center of the pumpkin pie filling reaches 175°. The center will still have a little wiggle, but it won't be super runny.
Serve with ice cream or whipped topping, if desired.
