Heat oven to 350°. Season chicken breast with garlic powder and pepper. Place in a baking dish and cover with foil. Bake 25 to 35 min, or until a thermometer inserted into the center reads 165°. Remove chicken and let cool before shredding.
In a large bowl, smash avocado. Stir in yogurt, lime juice, garlic powder and pepper. Stir in chicken, onion and celery. Store in an airtight container in the refrigerator. Serves 4