Marinate the Chicken: In a bowl, combine olive oil, lemon juice, dried oregano, garlic powder, paprika, salt, and pepper. Add the chicken thighs and coat them well. Let marinate in the refrigerator for at least 30 minutes, up to 2 hours for best flavor.
Prepare the Feta Tzatziki: In a mixing bowl, combine Greek yogurt, crumbled feta, grated cucumber, minced garlic, olive oil, lemon juice, fresh dill, salt, and pepper. Mix well and refrigerate for at least 20 minutes to allow flavors to meld.
Cook the Chicken: Preheat a grill or a skillet over medium-high heat. Remove the chicken from the marinade and cook for 6-7 minutes on each side or until fully cooked and golden brown, reaching an internal temperature of 75°C (165°F). Remove and let rest for a few minutes before slicing into strips.
Warm the Pita Breads: Lightly toast the pita breads on a grill or in a skillet for 1-2 minutes until heated through.
Assemble the Gyros: On each warmed pita bread, layer the sliced chicken, shredded lettuce, diced tomatoes, and cucumber. Drizzle with the prepared feta tzatziki.
Serve: Roll the pita around the filling tightly and serve immediately with extra tzatziki on the side.
Notes
For best flavor, marinate the chicken for up to 2 hours.