There's something a bit surreal about staging a Gershwin musical about 1920s New York high society in a park. It's hard to conjure the ambience of a sparkling house party with the wind gently soughing in the trees above the invitees' heads. And, let's face it, the Charleston just isn't an open-air sort of dance. But then the spirit of surrealism hovered over the jazz age so, on second thoughts, the incongruity may not be so inappropriate after all.