Preheat the oven to 350°F (175°C). Grease a 9×5-inch loaf pan or line it with parchment paper.
In a medium bowl, whisk together the oat flour, baking soda, baking powder, salt, cinnamon, nutmeg, ginger, and cloves until well combined.
In a large bowl, beat the granulated sugar, brown sugar, and eggs together until smooth and creamy.
Add the pumpkin puree, vegetable oil, Greek yogurt, and vanilla extract to the egg mixture and mix until fully incorporated.
Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined. Do not overmix.
Fold in the grated zucchini and chopped walnuts, if using, until evenly distributed.
Pour the batter into the prepared loaf pan and smooth the top with a spatula.
Bake in the preheated oven for 55 to 60 minutes, or until a toothpick inserted into the center comes out clean.
Remove the bread from the oven and let it cool in the pan for 10 minutes.
Transfer the bread to a wire rack and allow it to cool completely before slicing.