Gibson will take over the anchor chair May 29 and pull double duty with "Good Morning America" until June 30, ABC said on its Web site.
Pregnant "World News Tonight" co-anchor Elizabeth Vargas will go on maternity leave from the network and return as co-anchor of "20/20" and ABC News specials, the network said.
She was elevated to the nightly news anchor position along with Bob Woodruff in December 2005. Woodruff is still recovering from injuries he suffered when a roadside bomb exploded while he was reporting in Iraq in January and has no timetable for returning to work.
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Polly Bergen, Emmy Award-winning actress, dies