Gossip Girl actress Jessica Szohr made a podcast talking about gossip girl, she had executive producers Josh Schwartz and Stephanie Savage on as guests. The producers mentioned that Kristen Bell almost wasn't the voice of Gossip Girl. They were wary to hire her as she was already on Veronica Mars and also had a voice over on that show. They thought the same actress doing a voice over on two different shows on the same network didn't make sense. When they reazlied Veronica Mars wasn't going to come back, they snatched her up as fast as they could.
At least two other actresses were considered for the role before Kristen was cast.
-Selma Blair
-Christina Ricci (who apparently did the audio books of gossip girl)
Source ONTD, do you like Kristen Bell as the voice of gossip girl? Who would you have cast?