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 it sit for at least 15 minutes to absorb the flavors.
Grill the chicken thighs for about 6-7 minutes on each side, or until they reach an internal temperature of 165°F. Remove from the grill and let them rest for 5 minutes before slicing.
While the chicken is resting, prepare the bowls. Divide the cooked brown rice among four bowls.
Top each bowl with sliced grilled chicken, cherry tomatoes, diced cucumber, and red onion.
Drizzle tzatziki sauce over the top and sprinkle with crumbled feta cheese and chopped parsley.
Serve immediately and enjoy your delicious Greek chicken gyro bowls!