You Must Perform a Sim Swap.
A sim swap is the process of moving your number and your balance also your existing goodybag/gigabag to another giffgaff sim.
You could need this if you lost or have broken your sim or if you are getting a new phone.
Here is how to do it. 😉
1: Back up any contacts or anything that may be stored on the sim that you want to keep.
2: Visit the replace your sim section here https://giffgaff.com/profile/details#simswap
3: Enter the 6 digit activation code of the new SIM card, and click 'Continue'.
4: Select yes for you to commence sim swap. Then select yes I’m sure. This may take anything from a few minutes to 24 hours. This is normally around 30 minutes.
5: Put the new SIM into your phone. Turn the phone off/on at 30 minute intervals until you get signal and the SIM starts working.
6: Enjoy! You have successfully completed sim swap!
If any more information is needed then here is a guide http://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084
Hope this helped! 😉