In a bowl, mix olive oil, garlic powder, paprika, oregano, salt, and pepper.
Add sliced chicken breast to the marinade and coat well.
Let the chicken marinate for at least 30 minutes in the refrigerator.
Preheat a grill or skillet over medium-high heat.
Cook the chicken slices for about 5-7 minutes on each side until cooked through.
Remove the chicken from heat and let it rest for a few minutes.
Slice the chicken into strips.
Warm the pita bread on the grill for about 1 minute on each side.
Spread tzatziki sauce on the pita.
Layer the chicken, tomatoes, onions, and lettuce on top of the sauce.