Strain liquid from 1 cup shredded cucumber. Stir together shredded cucumber and tzatziki dressing in small bowl; cover and refrigerate until ready to serve.

For salad mixture, place remaining ½ cup chopped cucumber, tomato, olives, Italian salad dressing, and 1½ teaspoons dill in medium bowl; toss to coat and set aside.

Preheat a charcoal or gas grill for direct cooking over medium-high heat (375°). Spray a large grill basket with cooking spray; set aside.

Pat fish dry with paper towels. Season both sides with Greek seasoning. Place fish fillets in a single layer in prepared grill basket. Grill fish for 10 to 12 minutes or until fish reaches 145°, turning halfway through. Place pita breads on grill rack; grill 30 to 60 seconds or until warm, turning halfway through.

To serve, flake fish. Spoon some of the tzatziki mixture on pitas; top with fish, salad mixture, and remaining tzatziki mixture. Garnish with additional dill, if desired. Fold in half to serve.