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 sides down, in a large hot nonstick skillet. Cook over medium heat 6 minutes or until golden.
Spread remaining butter on ungrilled sides; turn and cook 6 more minutes or until golden. Repeat, if necessary, for any remaining sandwiches.
Oven-Grilled Loaded Turkey Melts: 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.