1. Preheat the oven to 370 degrees Fahrenheit (175 degrees Celsius) and set an oven rack in the middle position. Lightly grease an eight-by-four-inch (20-by-10-centimeter) loaf pan with butter or nonstick cooking spray. Dust with flour, shaking off any excess.
2. To make the cake, mix the flours, xanthan gum, poppy seeds, light brown sugar, baking powder, baking soda and salt in a medium bowl.
3. In a separate bowl, mix the orange juice, oat milk, oil, citrus zest and vanilla well to combine. Pour the liquid mixture into the flour mixture and mix using a spatula or wooden spoon.
4. Pour the batter into the prepared loaf pan and bake for 30 to 35 minutes, until golden brown on top and a toothpick inserted in the middle comes out clean. Let the cake cool for five minutes in the pan, then transfer to a wire rack and let it cool completely before adding the cream cheese icing.