Drain the canned tuna thoroughly and transfer it to a medium mixing bowl.Flake the tuna with a fork until evenly broken apart.Add the mayonnaise, Greek yogurt, Dijon mustard, lemon juice, garlic powder, salt, and black pepper. Mix until creamy.Stir in the diced celery, red onion, parsley, and pickle relish if using.Mix gently until all the ingredients are evenly combined.Cover and refrigerate for 10 minutes to allow the flavors to meld.Serve chilled in sandwiches, wraps, lettuce cups, over mixed greens, or with crackers.