In a large bowl, combine olive oil, minced garlic, lemon juice, oregano, cumin, paprika, salt, and pepper. Mix well.

Add the chicken breasts to the marinade and ensure they are evenly coated. Cover and refrigerate for at least 1 hour, up to overnight.

While the chicken marinates, prepare the Creamy Feta Tzatziki by combining Greek yogurt, crumbled feta cheese, grated cucumber, garlic, dill, mint, lemon juice, salt, and pepper in a bowl. Mix until smooth and creamy. Cover and chill until ready to use.

Preheat a grill or grill pan over medium-high heat. Remove the chicken from the marinade and grill for 6-7 minutes per side or until fully cooked and nicely charred. Let the chicken rest for 5 minutes, then slice thinly.

Warm the pita or flatbreads on the grill for a minute or until soft and pliable.

To assemble the gyros, place a few slices of chicken onto the center of each pita. Add a generous dollop of Creamy Feta Tzatziki, then top with red onion, tomatoes, cucumber, and lettuce.

Roll the pita to enclose the filling and serve immediately.