Preheat the oven to 180 ℃ (356 ℉)
In a stand-alone mixer with the paddle attachment or a hand-held mixer, beat the butter at medium-high speed until light and fluffy (3 minutes). Then add the sugar and beat until fluffy (3 minutes).
Next, add the eggs one at a time and mix until combined. Then add the vanilla extract, orange zest, lemon zest, and fresh orange juice and beat until combined.
In a separate bowl, mix the flours, baking powder, salt, baking soda, and xanthan gum using a whisk.
Add one-third of the dry ingredients to the wet ingredients, followed by half the buttermilk, and mix at low speed. Keep adding your dry ingredients followed by buttermilk until your run out of dry ingredients. Making sure that you finish with the dry ingredients.
Grease your bundt pan with butter and add your cake batter until the pan is ⅔ full.
Place your bundt pan in the oven on the center rack and bake it for 30-33 minutes or until a toothpick inserted at the middle comes out clean.