1. In a mixing bowl, combine olive oil, garlic, oregano, paprika, salt, and black pepper. Add chicken slices and mix well. Marinate for at least 30 minutes.
2. Heat a grill pan over medium-high heat. Grill the marinated chicken slices for about 5-7 minutes on each side or until fully cooked.
3. Warm the pita bread on the grill pan for about 1 minute on each side.
4. Assemble the sandwiches by placing grilled chicken, lettuce, tomato, cucumber, and red onion inside each pita bread. Drizzle with tzatziki sauce.