In a large bowl, combine olive oil, oregano, garlic powder, salt, and pepper. Add chicken thighs and coat well. Cover and marinate for at least 30 minutes or overnight.

Heat a skillet over medium-high heat. Add olive oil and marinated chicken. Cook for 5-7 minutes per side until golden brown and internal temperature reaches 165°F (75°C). Remove and let rest.

In a mixing bowl, combine Greek yogurt, crumbled feta cheese, lemon juice, and minced garlic. Stir until smooth and creamy, then season with salt and pepper to taste.

Chop cherry tomatoes, slice cucumber, and finely slice red onion. Arrange prepared vegetables on a plate for easy assembly.

Warm pita bread. Spread tzatziki, then add sliced chicken, cherry tomatoes, cucumber, and red onion. Roll pita tightly around the filling.

Slice the gyros in half, arrange on a platter, and serve immediately with extra tzatziki for dipping.