Preheat the oven to 350F and grease a standard loaf pan with gluten free cooking spray. Line with parchment so it overhangs the longer side edges by a couple of inches, and grease again. Set aside.
Make the cake. Whisk the brown sugar, oil, eggs, and pumpkin puree together in a large bowl until smooth and glossy. Add the flours, salt, and baking soda and whisk again until thoroughly combined. Fold in the candied ginger.
Pour the batter into the prepared baking pan and bake for 65-70 minutes, or until a toothpick inserted in the center comes out clean. This is a very tall loaf so make sure to use a 1-pound loaf tin.
Remove the cake from the oven and cool in the pan for 5 minutes. Run a knife around the edges of the cake and lift the cake out of the pan using the parchment paper overhang and set on a cooling rack. Cool completely before frosting.
Make the frosting. Whisk the butter and cream cheese until fluffy. Add the powdered sugar and heavy cream. Whisk again until smooth. Whisk in the orange zest and vanilla. Spread over the top of the cake and sprinkle with candied ginger and additional orange zest, if desired.
Notes
This cake holds up well so you can make it a day or two in advance. Store, loosely wrapped, in the refrigerator.