Preheat oven to 450 degrees F. Lightly oil a baking sheet or coat with nonstick spray.
GARLIC MIXTURE: In a small bowl, combine garlic, oregano, coriander, thyme, cumin, cayenne pepper, cinnamon, 1 1/4 teaspoons salt and 1 teaspoon pepper.
Place chicken in a single layer onto the prepared baking sheet. Stir in olive oil and GARLIC MIXTURE; gently toss to combine.
Place into oven and bake for 25 minutes, or until the chicken is completely cooked through, stirring halfway. Stir in lemon juice.
To serve, warm pitas in oven, about 4-5 minutes. Halve pitas, and fill with chicken, romaine lettuce, tomatoes, red onion, cucumber, feta and tzatziki sauce.