Preheat your oven to 350°F (175°C). Grease and flour a cake pan of your choice.
In a large mixing bowl, combine the gluten-free all-purpose flour and one spoon of baking soda. Stir well to distribute evenly.
In a separate bowl, whisk together the melted and cooled coconut oil (or butter), egg whites, and maple syrup until smooth.
Gently fold the pumpkin puree into the wet mixture until evenly distributed.
Gradually pour the wet ingredients into the dry ingredients, stirring continuously to form a smooth batter. Do not overmix.
Fold in the raisins, shredded coconut, and chopped walnuts until evenly spread throughout the batter.
In a small bowl, combine the lemon juice with the remaining half spoon of baking soda. Let it fizz briefly, then fold it into the batter.
Pour the batter into the prepared cake pan and spread evenly. Bake for 45 to 50 minutes, or until a toothpick inserted into the center comes out clean.
While the cake is still warm, drizzle the top with additional maple syrup. Let the cake cool completely in the pan before removing.