In a medium bowl, whisk together olive oil, oregano, garlic powder, salt, and pepper until combined. Add the chicken thighs and marinate for at least 30 minutes, ideally overnight.

Heat a skillet over medium-high heat for about 2 minutes. Cook the marinated chicken thighs for 5-7 minutes on each side until golden brown and cooked through.

In a separate bowl, combine Greek yogurt, feta, lemon juice, minced garlic, salt, and pepper. Mix until smooth and adjust seasoning as needed.

Slice the cooked chicken and assemble the gyros by spreading tzatziki on warm pita, adding chicken, cherry tomatoes, cucumber, and red onion. Roll tightly to serve.