Step 1: Marinate the Chicken

1. In a large bowl, combine the yogurt, olive oil, lemon juice, garlic, oregano, cumin, paprika, coriander, salt, and black pepper. Mix well.
2. Add the chicken thighs or breasts to the marinade, ensuring each piece is fully coated. Cover the bowl with plastic wrap and refrigerate for at least 1 hour, preferably 2-3 hours, to allow the flavors to penetrate the meat.

Step 2: Prepare the Tzatziki Sauce

1. Grate the cucumber and squeeze out as much water as possible using a cheesecloth or clean kitchen towel.
2. In a medium bowl, mix the Greek yogurt, grated cucumber, lemon juice, olive oil, garlic, dill, salt, and black pepper. Stir until well combined. Cover and refrigerate until ready to use, allowing the flavors to meld.

Step 3: Cook the Chicken

1. Heat a grill pan, skillet, or outdoor grill over medium-high heat. Lightly grease the surface with a drizzle of olive oil.
2. Remove the chicken from the marinade and cook for 5-7 minutes on each side, or until the internal temperature reaches 165°F (74°C). Avoid overcrowding the pan—cook in batches if necessary.
3. Once cooked, let the chicken rest for 5 minutes before slicing it into thin strips.

Step 4: Warm the Pita Bread

1. Brush each pita bread lightly with olive oil.
2. Heat a skillet over medium heat and warm the pita bread for 1-2 minutes on each side, until soft and pliable.

Step 5: Assemble the Gyros

1. Place a warm pita bread on a flat surface. Spread a generous layer of tzatziki sauce in the center.
2. Add sliced chicken, tomatoes, cucumbers, red onions, and crumbled feta (if using).
3. Fold the pita around the fillings, wrapping it tightly to form a gyro. Serve immediately.