Preheat the oven to 350°F (175°C). Lightly coat a 9 × 9-inch baking dish, or another similarly sized dish, with nonstick cooking spray.
Place the pumpkin purée, almond milk, egg, maple syrup, and vanilla extract in a large mixing bowl. Whisk thoroughly until the mixture is smooth and evenly blended.
Add the rolled oats, diced apple, baking powder, sea salt, cinnamon, ginger, allspice, and cloves to the pumpkin mixture. Stir until the oats are fully moistened and the apple pieces are distributed throughout.
Transfer the oatmeal mixture to the prepared baking dish. Use a spatula to spread it into an even layer.
Combine the granulated sugar and cinnamon in a small bowl. Scatter the cinnamon-sugar mixture uniformly across the surface of the oatmeal.
Bake for 30–35 minutes, or until the center is set and the top has developed a light golden color.
Remove the dish from the oven and allow the baked oatmeal to stand for 10 minutes before slicing and serving.