Preheat your oven to 350°F (175°C).

Grease a 9x13-inch baking pan lightly with cooking spray or butter, or line it with parchment paper.

In a medium bowl, whisk together the gluten-free all-purpose flour, baking powder, baking soda, ground cinnamon, nutmeg, ginger, and salt.

In a large bowl, combine the granulated sugar, brown sugar, and vegetable oil. Mix until well combined. Then, add the pumpkin puree, eggs, and vanilla extract. Stir until smooth and creamy.

Gradually add the dry ingredients to the wet ingredients. Stir gently until just combined. If adding nuts or chocolate chips, fold them in at this stage.

Pour the batter into the prepared baking dish and spread it evenly across the pan.

Bake for 25-30 minutes. Check for doneness with a toothpick; if it comes out clean, the bars are ready.

Let the bars cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely. Cut into squares and serve.