Decked out in a green gingham dress topped with an overcoat and tiny top hat of the same color, Cheryl Rogers proclaimed herself the “world’s oldest munchkin.” “I’m sort of the grandma of the group,” she joked, as her plastic emerald necklace caught the stage light during a recent dress rehearsal of Gypsy Theatre Company’s production of “The Wizard of Oz.” The show began its three-week run at the Cumming Playhouse on Thursday.