1. In a mixing bowl, combine olive oil, oregano, garlic powder, paprika, salt, and black pepper.
2. Add the sliced chicken breast to the bowl and coat well with the marinade. Let it sit for at least 15 minutes.
3. Preheat the grill to medium-high heat.
4. Grill the chicken for about 5-7 minutes on each side, or until fully cooked.
5. Warm the pita bread on the grill for about 1 minute on each side.
6. Assemble the gyros by placing grilled chicken, lettuce, tomatoes, red onion, and tzatziki sauce on each pita bread.
7. Fold the pita bread and serve immediately.