Prepare the marinade. In a large bowl, combine Greek yogurt, olive oil, lemon juice, garlic and all spices. Mix well.
Add chicken to the bowl. Coat with the marinade. Cover and refrigerate for 30 minutes to overnight.
Add the chicken to your crockpot. Cook on low for 3-4 hours, or until the internal temperature reaches 165°F and the meat is tender. Chicken breasts can dry out easily. Do not overcook.
When cooked, remove chicken from the crockpot. Shred the chicken with two forks. Save some of the crockpot liquid to add back to the chicken for more moisture, if you desire.
In a small bowl, mix together all of the ingredients for the garlic sauce.
Serve chicken over a pita. Top with tomatoes, cucumbers, feta cheese, and garlic sauce. Enjoy!