Preheat the grill pan or outdoor grill over medium heat.
In a mixing bowl, combine olive oil, garlic powder, paprika, salt, and black pepper. Add the chicken breasts and coat them evenly with the marinade.
Once the grill is hot, add the chicken breasts and cook for about 6-7 minutes on each side, or until the internal temperature reaches 165°F (75°C). Remove from grill and let rest for a few minutes.
While the chicken is resting, prepare the pitas by slicing them in half to create pockets.
Thinly slice the grilled chicken into strips.
To assemble, fill each pita pocket with the sliced chicken, chopped romaine lettuce, diced tomatoes, cucumber slices, and red onion.
Drizzle tzatziki sauce inside each pita before serving.