In a medium glass bowl, combine yogurt, garlic and lemon juice; add chicken and stir to coat. Cover and refrigerate for 2 to 3 hours.
Preheat oven to 400ºF. Coat a non-stick baking sheet with cooking spray. Remove chicken from refrigerator; scrape off excess yogurt and place chicken on prepared baking sheet. Bake until cooked through, flipping once, about 20 minutes.
Meanwhile, combine sauce ingredients in a small bowl; stir well.
Make a slit in each pita and fill each with equal amounts chicken, tomato and cucumber; spoon in sauce. Yields 1 pita per serving.