In a large mixing bowl, combine the shredded chicken, diced apple, celery, grapes, and walnuts (if using).
In a separate small bowl, whisk together the Greek yogurt, mayonnaise, Dijon mustard, lemon juice, salt, black pepper, and garlic powder until smooth.
Pour the dressing over the chicken mixture and stir gently until everything is well coated.
Taste and adjust seasoning if necessary.
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld.
Serve chilled on a bed of greens, in a sandwich, or with crackers.