Preheat the grill or grill pan over medium-high heat.
Season the chicken breasts with olive oil, garlic powder, oregano, salt, and pepper.
Grill the chicken for about 6-7 minutes on each side or until fully cooked, with an internal temperature of 165°F (75°C). Remove from the grill and let it rest for a few minutes.
While the chicken is resting, prepare the sauce by mixing Greek yogurt, lemon juice, garlic powder, salt, and pepper in a bowl.
Once the chicken has rested, slice it into thin strips.
Cut the pita breads in half to form pockets.
Fill each pita pocket with a layer of lettuce, followed by the grilled chicken, cherry tomatoes, cucumber, and red onion.
Drizzle the creamy dressing over the fillings and serve immediately.