In a large plastic zip-top bag, combine the garlic, vinegar, lemon juice, yogurt, olive oil, oregano, cumin, granulated garlic, thyme, marjoram (if using), pepper, and salt. Add the chicken and massage until the chicken is thoroughly coated. Let marinate at room temperature for 10-15 minutes or refrigerated for up to 12 hours.
Preheat the grill on medium-low. Remove the chicken from the marinade, allowing any excess to drip off. Place the chicken on the grill and cook until the chicken begins to lightly char, about 4-6 minutes per side. Continue cooking, turning occasionally, until the internal temperature of the chicken reads 165°F on an instant-read thermometer, about 10 minutes more. Transfer to a cutting board and let rest for at least 5 minutes, then thinly slice.
To serve, top the pitas with the chicken. Garnish your choice of the tzatziki, tomatoes, cucumbers, onion, olives, feta, and/or romaine.