Preheat the oven to 350°F. In a large bowl, combine the oats, 1/2 cup flour, pecans, light-brown sugar, ginger, salt, and butter. Mix until large crumbs form, and refrigerate until ready to use. In a 9-by-13-inch baking dish, combine the pumpkin, apples, lemon zest and juice, sugar, cinnamon, and remaining 2 tablespoons flour; toss until mixed. Sprinkle the oat crumble over the apple mixture and bake until the topping is browned and juices are bubbling, about 45 minutes. Let stand for 15 minutes. Serve warm with vanilla ice cream.