THE artist Cindy Sherman stood underneath “Bangkok II,” a new large-scale photograph by Andreas Gursky that was being unveiled Nov. 4 at the fortress-like Gagosian Gallery in Chelsea. Ms. Sherman stared in amusement as Mr. Gursky and Klaus Biesenbach, the MoMA P.S. 1 director, fawned over her sequined Marni handbag. “Oooh, nice,” cooed Larry Gagosian, the super gallerist. “Are you coming by later?”