In a large mixing bowl, combine the diced chicken, chopped celery, and halved grapes.
In a separate small bowl, whisk together the mayonnaise, Greek yogurt, Dijon mustard, lemon juice, garlic powder, salt, and pepper until smooth.
Pour the dressing over the chicken mixture and gently stir until everything is well coated.
If using, fold in the chopped walnuts and parsley for added crunch and flavor.
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld.
Serve chilled on a bed of lettuce, in a sandwich, or with crackers.