The actress is perhaps best known for her role on Lost, where she played Nadia, the love interest of Naveen Andrews's Sayid.
Variety reports that Gabriel will play a businesswoman from Dubai on the New York soap.
It was recently announced that Michelle Trachtenberg will also be returning for the last season of the show.
The daughter of actors Sandy and John Gabriel, she will next be seen later this year in Breaking Dawn Part 2.
Season six of Gossip Girl premieres Monday, October 8 at 9pm in the US.