To a bowl add in the yogurt, garlic, lemon zest, lemon juice, salt and pepper whisk well.
Add the chicken into the bottom of the slow cooker add in ⅓ of the greek yogurt sauce in the slow cooker. (Refrigerate the rest for later)
Stir to coat the chicken in the sauce in the slow cooker.
Cover with a lid and let the chicken and cook on high for 3-4 hours or on low for 5-6 until the chicken is tender and cooked through.
Remove the chicken from the slow cooker and place onto a plate. Discard the cooking liquid in the slow cooker. Shred the chicken with two forks. Add the chicken back to the slow cooker, then pour over ⅓ cup of the reserved Greek sauce marinade, tossing to coat. Cook on low for 5 more minutes.
Serve over pita with diced veggies with the remaining ⅓ cup Greek sauce marinade on top or for dipping on the side.