1. In a mixing bowl, combine olive oil, oregano, garlic powder, paprika, salt, and black pepper. Add chicken slices and coat well. Let it marinate for at least 15 minutes.
2. Preheat the grill to medium-high heat. Grill the chicken for about 5-7 minutes on each side until fully cooked.
3. While the chicken is grilling, prepare the tzatziki sauce. In a bowl, mix Greek yogurt, lemon juice, minced garlic, chopped dill, and grated cucumber. Stir well.
4. Warm the pita bread on the grill for about 1 minute on each side.
5. Assemble the gyros by placing grilled chicken slices on the pita bread, adding diced tomato, sliced red onion, and a generous amount of tzatziki sauce. Serve immediately.