Stir together avocado, mayonnaise, and garlic powder. Spread avocado mixture on one side of 4 bread slices. Top evenly with turkey, tomato, onion, bacon, cheese, and remaining bread slices.
Spread half of butter on one side of each sandwich. Place buttered sandwiches on a baking sheet. Place a second baking sheet on top of sandwiches. Bake at 400° for 20 minutes or until golden.