Prepare the slow cooker liner with nonstick cooking spray.
Place chicken breast into the bottom of the slow cooker. Pour the chicken stock over the chicken. Sprinkle the chicken with garlic powder, salt, and black pepper.
Cut the cream cheese into cubes and place over the top of the chicken.
Cover the slow cooker and cook on high for 2 to 3 hours or low for 4 to 5 hours.
Carefully remove the chicken from the slow cooker. Cut into slices, shred, or cube.
Whisk the remaining contents of the slow cooker until the cream cheese is blended with the juices.
To the slow cooker, add the warmed heavy cream and cheeses. Stir in the cheeses and continue stirring until melted.
Sprinkle the sauce with xanthan gum and stir to combine. The xanthan gum will thicken slightly and then continue to thicken as the sauce begins to cool.
If desired, return the chicken to the slow cooker.
Serve the chicken and sauce over zoodles for a low carb option or over regular fettuccine noodles.