Preheat the grill or grill pan to medium-high heat. Sprinkle steak evenly with 1/4 teaspoon salt, 1/4 teaspoon garlic powder, and 1/4 teaspoon pepper. Place steak on grill rack coated with cooking spray; 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 garlic powder, and remaining 1/4 teaspoon pepper 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 yogurt mixture, 1/4 cup onion mixture, and 1 halved tomato slice.