Mix all of the ingredients except the chicken in a large bowl or rectangular baking dish. Add the raw chicken, toss to coat, and marinade, covered in the refrigerator , for at least 1 hour, up to 4 hours.
Grill the chicken thighs until charred on both sides and internal temperature is 165-175 degrees. Place on a cutting board and chop/shred the chicken into pieces.
Wrap the pita bread in a damp kitchen towel. Place in microwave and heat for 30 seconds until warm and pliable.
Assemble the gyros : Fold a pita in half and put some tzatziki sauce on the bottom. Fill it with chicken then top with more tzatziki sauce. Top with preferred toppings of lettuce, diced tomatoes and/or diced onions.
Enjoy!