In a bowl, combine the Greek yogurt, minced garlic, olive oil, lemon juice, oregano, paprika, salt, and black pepper. Mix well to form a marinade.
Add the chicken thighs to the marinade, ensuring they are well-coated. Cover the bowl and let it marinate in the refrigerator for at least 30 minutes, or up to overnight for maximum flavor.
Preheat the grill or frying pan over medium-high heat.
Once heated, remove the chicken from the marinade and cook for about 5-7 minutes on each side, or until the chicken is cooked through and has nice grill marks. The internal temperature should reach 165°F (75°C).
Remove the chicken from the heat and let it rest for a few minutes before slicing it into strips.
To assemble, place a few strips of chicken in the center of each pita bread. Top with diced tomatoes, cucumber, sliced red onion, and a generous dollop of tzatziki sauce.
Garnish with fresh parsley and serve immediately.