Marinate the chicken: mix all ingredients for the marinade in a large bowl and stir in the chicken thigh filets. Cover with plastic wrap and refrigerate for 2 hours or overnight.
Grate the cucumber and place it in a colander to remove the excess water. Press it down to dry it as much as you can.
Prepare the tzatziki by combining the grated cucumber with the rest of the ingredients.
For extra topping, make a salad by combining sliced grape tomatoes, chopped cucumber, sliced red onion, chopped fresh parsley, and mint. Season with salt and pepper.
Grill the chicken: remove any excess marinade from the chicken thighs and grill them in a preheated outdoors grill or a hot grill pan.
Assembling the gyros: warm the pita bread in the microwave, place a lettuce leaf and a sliced grilled chicken thigh in the middle of the warm pita, top with tzatziki, salad, and fold.