In a small skillet, cook bacon over medium until crisp, 10 minutes. With tongs, transfer bacon to paper towels to drain. Add chicken to skillet and stir to coat with drippings. Season with salt and pepper. In a small bowl, mix together mayonnaise and garlic; season with salt and pepper. Spread mayonnaise on bread and layer with bacon, chicken, avocado, and lettuce. Top with the other slice of bread.