"It has not been an easy decision to make," Gibson wrote. "This has been my professional home for almost 35 years. And I love this news department, and all who work in it, to the depths of my soul.
"The proudest part for me has been saying '...for all of us at ABC News...,' since those words signify in my mind that I have been in a position to speak for an entire news department that I consider second to none," he added.
In a separate e-mail to the staff at the time of Gibson's announcement, Westin acknowledged Gibson's "leadership" during "a difficult and turbulent time both for ['World News'] and for ABC News overall."
"Having accomplished so much in so many different parts of ABC News, Charlie has decided it is time for him to step down," Westin wrote. "I have told him that he has an open door to continue to work with ABC News, but he's asked for a bit of time before he comes back to us."
This month, Jon Banner, executive producer of "World News with Charles Gibson," revealed that Gibson's last day as "World News" anchorman would be today, Dec. 18.
"We are preparing to see our very good friend and anchor off as he begins a new journey," Banner wrote, as he announced plans for a series of tributes and retrospective segments that have been broadcast on "World News" this week.
In the segments, Gibson has recalled the full sweep of the news and history he has covered for ABC News -- including civic unrest in the streets, interviews with presidents and other pre-eminent politicians and encounters with everyday people he considered heroes.
"It's a pretty great profession that gets you inside the gates," he said this week on "World News."
A native of Evanston, Ill., Gibson grew up in Washington, D.C.
Gibson's career in journalism has spanned at least five decades, even extending back to his time as a student at Princeton University, where he was news director for a college radio station, WPRB-FM.
As part of Thursday's celebration, Westin presented Gibson with a map of the world drawn in 1748. Westin noted the importance of the date -- two years after Princeton was founded, ensuring Princeton was on this historical map. Gibson currently serves as a trustee of Princeton and will work in that capacity until next year.
Gibson's first professional broadcasting job was as Washington producer for RKO Network in 1966.
He came to ABC News in the 1975. Before settling in as co-anchor of "Good Morning America," he worked as a correspondent on "ABC's World News Tonight With Peter Jennings," as an occasional substitute anchor for Ted Koppel on "Nightline," and as substitute anchor on "World News This Morning."
Gibson co-anchored "Good Morning America" from 1987 to 1998 and 1999 to 2006 before taking the "World News" helm.
As he wrapped up his speech Thursday, Gibson told his colleagues it is important to maintain their same high standards after he leaves.
"It is important, what we do," he said. "We need to keep doing it in an objective and fair manner."