Place beef between 2 sheets of heavy-duty plastic wrap; flatten to 1/2-inch thickness using a meat mallet or rolling pin.
Combine garlic and next 3 ingredients in a shallow dish or large heavy-duty zip-top plastic bag; add beef. Cover or seal, and chill 8 hours. Remove beef from marinade, discarding marinade.
Stir together yogurt and next 3 ingredients. Set aside.
Grill beef, covered with grill lid, over medium-high heat (300° to 350°) 7 minutes on each side or to desired degree of doneness. Let stand 5 minutes. Cut into thin slices.
Place beef evenly down center of warm pita rounds. Top with yogurt mixture, lettuce, and tomato; roll up. Garnish, if desired, and serve immediately.