01 - Preheat oven to 350°F. Grease and line a 9-inch loaf pan with parchment paper.
02 - Combine cranberries, sugar, orange juice, and zest in a small saucepan. Cook over medium heat, stirring occasionally, until cranberries burst and mixture thickens, about 8 to 10 minutes. Allow to cool slightly, then puree until smooth.
03 - Whisk gluten-free flour, baking powder, baking soda, and salt together in a medium bowl.
04 - In a large bowl, beat softened butter and granulated sugar until light and fluffy. Incorporate eggs one at a time, then stir in vanilla extract.
05 - Mix sour cream and milk into the creamed mixture until fully combined.
06 - Gradually add dry ingredients to wet ingredients, mixing until just combined without overmixing.
07 - Pour half the batter into the prepared pan. Spoon half the cranberry mixture over it and gently swirl with a knife. Repeat layering with remaining batter and cranberry swirl.
08 - Bake for 40 to 50 minutes or until a toothpick inserted into the center comes out clean. Cool in pan for 10 minutes, then transfer to a wire rack to cool completely.
09 - Whisk powdered sugar with orange juice until smooth and pourable. Drizzle over cooled cake before serving.