Preheat the oven to 350°F (175°C). Lightly grease an 8-inch square baking dish with butter or nonstick cooking spray.
In a large bowl, combine the rolled oats, cinnamon, ginger, nutmeg, baking powder, and salt.
In a separate medium bowl, whisk together the pumpkin puree, whole milk, eggs, maple syrup, melted butter, and vanilla extract until smooth.
Pour the pumpkin mixture into the dry ingredients. Stir until all the oats are evenly moistened.
Fold in the chopped pecans, distributing them evenly throughout the oatmeal mixture.
Pour the mixture into the prepared 8-inch square baking dish. Spread it into an even layer and make sure the oats and liquid are evenly distributed.
Bake at 350°F (175°C) for 35 to 40 minutes. The oatmeal is ready when the edges are lightly golden, the center is set rather than liquid, and a knife inserted near the middle comes out mostly clean.
Remove from the oven and let the baked oatmeal rest for 10 minutes. This allows the remaining moisture to absorb and helps the oatmeal hold together.
Cut or spoon into 6 servings and serve warm.