Preheat the grill or grill pan to medium-high heat. Coat grill rack with nonstick cooking spray.
Sprinkle steak evenly with 1/4 teaspoon salt, 1/4 teaspoon black pepper, and 1/4 teaspoon garlic powder. Place steak on greased grill rack, and grill 10 minutes or until desired degree of doneness, turning after 6 minutes. Let stand 5 minutes. Cut steak diagonally into thin slices.
Place shredded cucumber in paper towels; gently squeeze to release moisture. Stir cucumber, remaining 1/4 teaspoon salt, remaining 1/4 teaspoon black pepper, and remaining 1/4 teaspoon garlic powder into yogurt.
Combine onion, dill, oil, and juice in a bowl; toss to coat.
Divide steak evenly among flatbread pockets; top each with about 2 tablespoons of the yogurt mixture, 1/4 cup onion mixture, and 2 tomato halves.