The hit BBC show, written and created by James Corden and Ruth Jones, will return to Barry and Essex nine years after the last episode aired.
James Corden tweeted a picture of the script for the one-hour Christmas special.
In a joint statement, Corden and Jones - who play Smithy and Nessa in the show - said: "Over the last ten years we've talked a lot about Gavin and Stacey - where they might be today and what their lives might look like.
"And so in secret we took the plunge and wrote this one hour special.
"We've loved revisiting Barry and Essex again, and bringing the characters back together has been a joy.
"We're so excited to get the chance to work with our fabulous cast and crew once more and to give fans of the show a festive treat this Christmas."
The show follows the lives of Essex boy Gavin, played by Matthew Horne, and Barry girl Stacey, played by Joanna Page, whose long-distance romance eventually blossomed into a relationship between the pair.
They will be joined in the special by Rob Brydon, Larry Lamb, Melanie Walters and Alison Steadman.
Charlotte Moore, director of BBC Content, said: "We can't wait to see what's happened to everyone over the last nine years, and what's next for one of the nation's favourite comedy families."