In a large mixing bowl, combine the shredded chicken and chopped hard-boiled eggs.
In a separate bowl, mix together the mashed avocado, Greek yogurt, Dijon mustard, lemon juice, garlic powder, salt, and pepper until smooth.
Pour the avocado mixture over the chicken and eggs, and gently fold until everything is well combined.
Add the chopped green onions and diced celery, and fold again to incorporate.
Taste and adjust seasoning if necessary.
Serve immediately on a bed of lettuce, in a sandwich, or in a wrap. For best flavor, chill in the refrigerator for at least 30 minutes before serving.