This has been posted before here: http://www.hotukdeals.com/freebies/free-cake-your-birthday-greggs-1893718
but its still active so thought its worth reposting for those that missed it.
Saw the deal the other day for the free festive bake and signed up, when creating my profile I just chose a random birthday which happened to be a couple of days away. Just received an email saying I've had a Birthday Treat added to my account. The email says that I can choose either a cupcake or a doughnut, however the in app reward states 'Choose any single item from our confectionery range, from cupcakes to doughnuts.' so sounds like you can get anything. You have a month from your 'birthday' to claim.
Also found that with the festive bake offer you can register a new account with a new email and claim again, so sounds like free lunches until this expires to me!
Please see the original festive bake offer for details on how to claim via the app - http://www.hotukdeals.com/freebies/free-festive-bake-via-greggs-app-2070775