That's a good idea (I think that some networks gave you some sort of perk on your birthday in the distant past).
Maybe try putting this idea in giffgaff Labs to see if it's viable?
Of course, there'll also be a bunch of people asking shortly to tell you how there's payback, and so no need for any other perks with giffgaff...
Please don't put it in Labs - it's been suggested more than once I think. Search & support an existing idea in there.
Edit: yep, 2 pages of existing ideas for you to choose from
A digital birthday badge on the profile?
I think I'd still rather keep my date of birth concealed from fraudsters. If giffgaff don't need it to be able to run my account, I'm not going to volunteer it just to get a birthday treat. It was a bit of a shock when the Google Doodle was a birthday cake on my birthday. (Did feel a bit special, though, for a few seconds until I realised it was only me who could see it...)