Preheat oven to 350°.
Combine first 4 ingredients in a medium bowl; set aside.
Cream together butter and sugar with an electric mixer. Add eggs; beat to combine.
Combine flour, baking power, xanthan gum, and salt in a medium bowl. Add flour mixture gradually to butter mixture, alternating with sour cream. Begin and end with flour mixture. Mix until well blended. Stir in vanilla.
Pour half of batter into well-greased 12-cup Bundt pan. Spoon apple-pecan mixture evenly over batter. Top evenly with remaining batter, using a spatula if necessary to spread out the batter.
Bake at 350° for 1 hour or until a pick inserted in the center comes out clean. Cool in pan for 10 minutes; remove to a wire rack and let cool completely.