Xbox has announced it's Games With Gold titles for December. The list can be seen below as well as the dates that each of the titles will be available.
Child of Eden — December 1 - 15
Marlow Briggs and the Mask of Death — December 16 - 31
Warhammer: End Times -- Vermintide — December 1-31
Back to the Future: The Game -- 30th Anniversary Edition — December 16 - January 15