Gummer, who appeared in the television series "Emily Owens M.D." and on stage in Les Liaisons Dangereuses, School for Lies and Uncle Vanya, will play the personal trainer to Aniston's character.
"Cake" is written by Patrick Tobin and will be directed by Daniel Barnz.
It is produced by Ben Barnz, Kristin Hahn, Courtney Solomon and Mark Canton.