Gibson is retiring. His last broadcast as anchor on "World News" will be Dec. 18. Sawyer starts Dec. 21.
The 63-year-old Sawyer has been on "GMA" for 10 years. She says: "I've calculated -- 2,881 shows. Roughly."
Sawyer made the announcement Monday. She says she hopes "GMA" viewers will "celebrate with us this week, and laugh with us this week, and look back a little with us this week."