Preheat the oven to 350 degrees F. Line a 9 inch square pan with parchment paper so that it hangs over two sides.
In a large bowl, mix the melted vegan buttery spread and sugars together. You can use a mixer or a spoon to do this.
Add the pumpkin puree, vanilla extract, water, and vinegar and mix well.
Add the gluten free flour, baking powder, baking soda, salt, nutmeg, cinnamon, and ginger. Mix well. The batter will be thick. If it seems very dry, and a little more water until you can stir it and spread it in the pan.
Put the batter into the prepared pan and smooth it out with the back of a spoon.
Bake at 350 degrees F for 28-32 minutes, or until a toothpick inserted in the middle of the cake comes out clean. Set the cake pan in a cooling rack to cool.
Make the molasses buttercream:
Beat the vegan buttery spread and molasses together until smooth. Stop the mixer and add some powdered sugar. Continue to mix, stopping to add more sugar and milk until the frosting is smooth and spreadable.
Once the cake is completely cooled, spread the frosting on top. Cover the leftover cake and store in the refrigerator.
Notes
Cover leftover cake with plastic wrap and store in the refrigerator. It should stay fresh for up to 5 days.If you aren't gluten free, you can go ahead and use regular all purpose flour.