Preheat your grill or grill pan over medium-high heat. In a bowl, combine olive oil, oregano, garlic powder, onion powder, salt, and pepper. Add the chicken thighs and coat them well with the marinade. Let them marinate for at least 15 minutes.
Grill the chicken for about 6-7 minutes on each side or until fully cooked and the internal temperature reaches 165°F. Remove from heat and let rest for 5 minutes before slicing.
While the chicken is resting, prepare the bowls. In each bowl, add 1/2 cup of cooked brown rice or quinoa as the base.
Top the rice or quinoa with sliced grilled chicken, cherry tomatoes, cucumber, and red onion.
Drizzle tzatziki sauce over the top and sprinkle with crumbled feta cheese. Garnish with fresh parsley.
Serve immediately and enjoy your Greek Chicken Gyro Bowls!