Charles Gibson's retirement last month has triggered a cascading wave of personnel changes. With Diane Sawyer set to replace Gibson on World News, ABC executives have been scrambling to find a replacement for her on Good Morning America. Women such as Ashleigh Banfield and Suze Orman want the job but the top brass prefer to pair a man with co-host Robin Roberts in order to restore the show's traditional male-female balance. It's likely that ABC will promote internally--and the short list is thought to include World News Saturday anchor David Muir, GMA Weekend co-host Bill Weir, GMA news anchor Chris Cuomo, and This Week anchor George Stephanopoulos, who is set to fill in for Sawyer for the rest of the week. ABC staffers were bewildered that executives didn't have someone waiting in the wings.