1. In a mixing bowl, combine olive oil, garlic powder, oregano, paprika, salt, and black pepper. Add chicken slices and coat well. Let it marinate for at least 15 minutes.
2. Heat a grill pan over medium-high heat. Cook the marinated chicken slices for about 5-7 minutes on each side or until fully cooked.
3. In another bowl, mix Greek yogurt, lemon juice, chopped dill, minced garlic, salt, and black pepper to make the tzatziki sauce.
4. Warm the pita bread on the grill pan for about 1-2 minutes on each side.
5. Assemble the gyros by placing chicken slices on the pita bread, adding diced tomatoes, sliced red onion, and cucumber. Drizzle with tzatziki sauce.
6. Serve immediately and enjoy!