1. In a mixing bowl, combine Greek yogurt, olive oil, minced garlic, paprika, ground cumin, salt, and black pepper. Mix well.
2. Add chicken breasts to the marinade, ensuring they are well-coated. Cover and refrigerate for at least 1 hour.
3. Preheat a grill pan over medium heat. Grill the marinated chicken breasts for about 6-7 minutes on each side, or until fully cooked.
4. Let the chicken rest for a few minutes, then slice into strips.
5. Warm the pita bread on the grill pan for about 1 minute on each side.
6. Assemble the sandwiches by placing lettuce, tomatoes, red onion, and grilled chicken strips inside each pita bread.